Cerebrospinal, Synovial, Serous Body Fluids, and Alternative Specimens

Xanthochromia

Xanthochromia commonly refers to a pale pink to yellow color in the supernatant of centrifuged CSF, although other colors may be present ( Table 30.1 ). To detect xanthochromia, the CSF should be centrifuged and the supernatant fluid compared with a tube of distilled water. Xanthochromic CSF is pink, orange, or yellow owing to RBC lysis and hemoglobin breakdown. Pale pink to orange xanthochromia from released oxyhemoglobin is usually detected by lumbar puncture performed 2 to 4 hours after the onset of subarachnoid hemorrhage, although it may take as long as 12 hours. Peak intensity occurs in about 24 to 36 hours and then gradually disappears over the next 4 to 8 days. Yellow xanthochromia is derived from bilirubin. It develops about 12 hours after a subarachnoid bleed and peaks at 2 to 4 days but may persist for 2 to 4 weeks.

Visible CSF xanthochromia may also be due to the following: (1) oxyhemoglobin resulting from artifactual RBC lysis caused by detergent contamination of the needle or collecting tube, or a delay of longer than 1 hour without refrigeration before examination; (2) bilirubin (bilirachia) in jaundiced patients; (3) CSF protein levels over 150 mg/dL, which are also present in bloody traumatic taps (>100,000 RBCs/μL) or in pathologic states such as complete spinal block, polyneuritis, and meningitis; (4) disinfectant contamination; (5) carotenoids (orange) in people with dietary hypercarotenemia (i.e., hypervitaminosis A); (6) melanin (brownish) from meningeal metastatic melanoma; and (7) rifampin therapy (red-orange).

Although careful gross CSF inspection has good sensitivity ( ), spectrophotometry can also help to differentiate hemoglobin-derived substances from other xanthochromic pigments with different maximal absorption peaks ( ).

Differential Diagnosis of Bloody CSF

A traumatic tap occurs in about 20% of lumbar punctures. Therefore, distinction of a traumatic puncture from a pathologic hemorrhage is of vital importance. Although the presence of crenated RBCs is not useful, the following observations may be helpful in distinguishing the two forms of bleeding.

• 1.

  In a traumatic tap, the hemorrhagic fluid usually clears between the first and third collected tubes but remains relatively uniform in subarachnoid hemorrhage.

• 2.

  Xanthochromia, microscopic evidence of erythrophagocytosis, or hemosiderin-laden macrophages indicate a subarachnoid bleed in the absence of a prior traumatic tap. RBC lysis begins as early as 1 to 2 hours after a traumatic tap. Thus, rapid evaluation is necessary to avoid false-positive results.

• 3.

  A commercially available latex agglutination immunoassay test for cross-linked fibrin derivative d -dimer is specific for fibrin degradation and should theoretically be negative in traumatic taps ( ). However, it has been shown to not effectively distinguish subarachnoid hemorrhage from traumatic lumber puncture ( ).

Total Cell Count

Although the traditional manual method for cell counting in CSF samples, using undiluted CSF in a manual counting chamber, continues to be a useful approach, because of the low cell counts frequently encountered in CSF, the precision of manual counting in these samples is inherently limited ( , ). Improvements in the hardware and software in flow cytometers now allow reliable use of these instruments in performing automated total WBC counts and WBC differential counts ( ; ), and even in detecting bacteria ( ) in CSF samples. Although these instruments are increasingly used clinically to perform these counts on CSF samples, the low clinical decision levels for total WBC count in CSF and persistent technical limitations of flow cytometers at low WBC levels and with certain WBC types continue to be cause for concern ( ; ; ; ). When issues of turnaround time and cost are considered, however, automated total WBC and WBC differential counts in CSF samples is a reasonable alternative to manual counting ( ; ), with added manual examination of the sample to avoid missing pathologic cell types as a useful cross-check ( ). When considering use of automated cell counters with CSF samples, laboratories should carefully follow manufacturer and Clinical and Laboratory Standards Institute guidelines ( ) when implementing these automated methods.

The normal leukocyte cell count in adults is 0 to 5 cells/μL. It is higher in neonates, ranging from 0 to 30 cells/μL, with the upper limit of normal decreasing to adult values by adolescence. No RBCs should be present in normal CSF. If numerous (except with a traumatic tap), a pathologic process is probable (e.g., trauma, malignancy, infarct, hemorrhage). Although RBC counts have limited diagnostic value, they may give a useful approximation of the true CSF WBC count or total protein in the presence of a traumatic puncture by correcting for leukocytes or protein introduced by the traumatic puncture. To be valid, all measurements (WBC, RBC, protein) must be performed on the same tube. This procedure also assumes that the blood is derived exclusively from the traumatic tap. The corrected WBC count is as follows:

where

and

An analogous formula may be used to correct for added total protein (TP):

In the presence of a normal peripheral blood RBC count and serum protein, these corrections amount to about 1 WBC for every 700 RBCs and 8 mg/dL protein for every 10,000 RBCs/μL. This latter RBC correction factor is reasonably accurate as long as the peripheral WBC count is not extremely high or low.

An observed/expected (added) WBC count ratio greater than 10 has a sensitivity of 88% and a specificity of 90% for bacterial meningitis. When the predicted WBC count is below the observed count, the probability of bacterial meningitis appears to be low ( ; ).

Differential Cell Count

Suggested differential count reference ranges are presented in Table 30.2 . A differential performed in a counting chamber is unsatisfactory because the low cell numbers give rise to poor precision and identifying the cell type beyond granulocytes and “mononuclears” is difficult in a wet preparation. Direct smears of the centrifuged CSF sediment are also subject to significant error from cellular distortion and fragmentation.

The cytocentrifuge method is rapid, requires minimal training, and allows Wright staining of air-dried cytospins. It is the recommended method for differential cell counts in all body fluids ( ). Cell yield and preservation are better than with simple centrifugation. From 30 to 50 cells can be concentrated from 0.5 mL of “normal” CSF. Variable artifactual distortions may be seen, but they are minimized when the specimen is fresh, albumin is added to the specimen (2 drops of 22% bovine serum albumin), and the cell concentration is adjusted to about 300 WBCs/L prior to centrifugation ( ). Manual differential cell counting on a cytocentrifuge preparation of CSF continues to be the most reliable method, even with low cell numbers. Although automated differential counts may be safely performed on CSF samples using flow cytometers ( ; ; ), confirmation of the automated differential count by manual examination of a cytocentrifuged smear is recommended with some instruments ( ; ) and is a requirement with specimens at risk for containing neoplastic cells. Effective detection of neoplastic cells, such as leukemic cells, using the cytocentrifuge method on CSF appears to be highly instrument dependent ( ).

Filtration and sedimentation methods are too cumbersome for routine use. However, filtration does allow concentration of large volumes of CSF for cytologic examination or culture while retaining the fluid filtrate for additional studies.

In adults, normal CSF contains small numbers of lymphocytes and monocytes in an approximate 70:30 ratio ( Fig. 30.1 ). A higher proportion of monocytes is present in young children, in whom up to 80% may be normal ( ). Erythrocytes due to minor traumatic bleeding are commonly seen, especially in infants. Small numbers of neutrophils (polymorphonuclear leukocytes [PMNs]) may also be seen in “normal” CSF specimens, most likely as a result of minor hemorrhage ( ) and improved cell concentration methods. No general consensus regarding an upper limit of normal for PMNs has been established. Many laboratories accept up to 7% neutrophils with a normal WBC count. Over 60% neutrophils has been reported in high-risk neonates without meningitis ( ). The number of PMNs may be decreased by as much as 68% within the first 2 hours after lumbar puncture owing to cell lysis ( ).

Traumatic puncture may result in the presence of bone marrow cells, cartilage cells, squamous cells, ganglion cells, and soft-tissue elements. In addition, ependymal and choroid plexus cells may rarely be seen ( Fig. 30.2 ). Moreover, blastlike primitive cell clusters, most likely of germinal matrix origin, are sometimes found in premature infants with intraventricular hemorrhage ( Fig. 30.3 ).

Increased CSF neutrophils occur in numerous conditions ( Box 30.3 ). In early bacterial meningitis, the proportion of PMNs usually exceeds 60%. However, in about one-quarter of cases of early viral meningitis, the proportion of PMNs also exceeds 60%. Viral-induced neutrophilia usually changes to a lymphocytic pleocytosis within 2 to 3 days. A total PMN count of over 1180 cells/μL (or more than 2000 WBCs/μL) has a 99% predictive value for bacterial meningitis ( ). Persistent neutrophilic meningitis (over 1 week) may be noninfectious or due to less common pathogens such as Nocardia , Actinomyces , Aspergillus , and the zygomycetes ( ).

Increased CSF lymphocytes have been reported in various diseases/disorders ( Box 30.4 ). Lymphocytosis (>50%) may occur in early acute bacterial meningitis when the CSF leukocyte count is under 1000/μL ( ). Reactive lymphoplasmacytoid and immunoblastic variants may be present, particularly with viral meningoencephalitis. Blastlike lymphocytes may be seen admixed with small and large lymphocytes in the CSF of neonates.

Plasma cells , not normally present in CSF, may appear in a variety of inflammatory and infectious conditions ( Box 30.5 ), along with large and small lymphocytes, and in association with malignant brain tumors ( ). Multiple myeloma may also rarely involve the meninges ( ).

Although eosinophils are rarely present in normal CSF, they may be increased in a variety of CNS conditions ( Box 30.6 ). For example, eosinophilia is frequently mild (1%–4%) in a general inflammatory response, but in children with malfunctioning ventricular shunts, it may be marked ( Fig. 30.4 ). A suggested criterion for eosinophilic meningitis is 10% eosinophils ( ); parasitic invasion of the CNS is the most common cause worldwide. Coccidioides immitis is a significant cause of CSF eosinophilia in endemic regions of the United States ( ).

Increased CSF monocytes lack diagnostic specificity and are usually part of a “mixed-cell reaction” that includes neutrophils, lymphocytes, and plasma cells. This pattern is seen in tuberculous and fungal meningitis, chronic bacterial meningitis (i.e., Listeria monocytogenes and others), leptospiral meningitis, ruptured brain abscess, Toxoplasma meningitis, and amebic encephalomeningitis. A mixed-cell pattern without neutrophils is characteristic of viral and syphilitic meningoencephalitis. Macrophages with phagocytosed erythrocytes ( erythrophages ) appear from 12 to 48 hours following a subarachnoid hemorrhage or traumatic tap. Hemosiderin-laden macrophages ( siderophages ) appear after about 48 hours and may persist for weeks ( Fig. 30.5 ). Brownish yellow or red hematoidin crystals may form after a few days.

Morphologic CSF examination for tumor cells has moderate sensitivity and high specificity (97%–98%) ( ). Sensitivity depends on the type of neoplasm. CSF examination of leukemic patients has the highest sensitivity (about 70%), followed by metastatic carcinoma (20%–60%) and primary CNS malignancies (30%). Sensitivity may be optimized by using filtration methods with larger fluid volumes or by performing serial punctures in patients in whom a neoplasm is strongly suspected. Processing of CSF samples using liquid-based thin-layer methods also increases sensitivity in the detection of neoplastic cells and enhances preservation of these cells for potential immunocytochemical analysis ( ). These liquid-based methods are now commonly used for cytopathologic examination of CSF and other body cavity fluid specimens.

Leukemic involvement of the meninges is more frequent in patients with acute lymphoblastic leukemia ( Fig. 30.6 ) than in those with acute myeloid leukemia ( Fig. 30.7 ). Both are significantly more common than CNS involvement in the chronic leukemias. A leukocyte count over 5 cells/μL with unequivocal lymphoblasts in cytocentrifuged preparations is commonly accepted as evidence of CSF involvement. The incidence of CNS relapse in children with lymphoblasts but cell counts lower than 6 cells/μL appears to be low and is not significantly different from cases in which no blasts are identified ( ; ; ). As noted previously, the ability to detect leukemic cells in CSF using the cytocentrifuge method is highly instrument dependent; low levels of these cells may be missed with certain instruments ( ).

Non-Hodgkin lymphomas involving the leptomeninges are usually high-grade tumors (lymphoblastic, large-cell immunoblastic, and Burkitt lymphomas; Fig. 30.8 ); low-grade lymphomas and Hodgkin lymphoma are significantly less common ( ; ). T cells predominate in normal and inflammatory conditions, whereas most lymphomas, especially those occurring in immunocompromised hosts, are of B-cell lineage. Lymphoblastic lymphoma, the most common T-cell lymphoma to involve the CSF, can be detected by terminal deoxynucleotidyl transferase stain.

Multiparameter flow cytometric immunophenotypic studies, deoxyribonucleic acid (DNA) analysis by polymerase chain reaction (PCR), and, more recently, DNA sequence analysis have been shown to significantly improve diagnostic sensitivity and specificity in CSF samples involved by leukemic or lymphoma cells ( ; ; ; ; ; ; , ). Considering their greater sensitivity in detecting leukemia or lymphoma cells in CSF samples, whenever available, these advanced technologies should be part of the complete evaluation in patients being worked up for these conditions.

Amebas, fungi (especially Cryptococcus neoformans ), and Toxoplasma gondii organisms may be present on cytocentrifuge specimens but may be difficult to recognize without confirmatory stains.

Proteins

Albumin and IgG Measurements

The permeability of the BBB may be assessed by immunochemical quantification of the CSF albumin/serum albumin ratio in grams per deciliter (g/dL). The normal ratio of 1:230 ( ) yields an unwieldy decimal of 0.004, which prompted the use of the CSF/serum albumin index , which is arbitrarily calculated as follows:

$\text{CSF}/\text{Serum}\text{albumin}\text{index}=\frac{\text{CSF}\text{albumin}\left(\text{mg}/\text{dL}\right)}{\text{Serum}\text{albumin}\left(\text{g}/\text{dL}\right)}$

An index value less than 9 is consistent with an intact barrier. Slight impairment is considered with index values of 9 to 14, moderate impairment with values of 14 to 30, and severe impairment with values greater than 30 ( ). The index is slightly elevated in infants up to 6 months of age, reflecting the immaturity of the BBB, and increases gradually after 40 years of age. A traumatic tap invalidates the index calculation.

Increased intrathecal IgG synthesis is reflected by an increase in the CSF/serum IgG ratio:

$\text{CSF}/\text{Serum IgG ratio}=\frac{\text{CSF}\text{IgG}\left(\text{mg}/\text{dL}\right)}{\text{Serum}\text{IgG}\left(\text{g}/\text{dL}\right)}$

The normal ratio is 1/390 or 0.003 ( ). Similar to the albumin index, the CSF/serum IgG index may be obtained by using milligrams per deciliter for the CSF IgG value. The CSF/serum IgG index normal range is 3.0 to 8.7.

The CSF/serum IgG index can be elevated by intrathecal IgG synthesis or increased plasma IgG crossover from breakdown of the BBB. Ig derived from plasma crossover may be corrected by dividing the CSF/serum IgG index by the CSF/albumin index to yield the CSF IgG index.

$\text{CSF}\text{IgG}\text{index}=\frac{\text{CSF}\text{IgG}\left(\text{mg}/\text{dL}\right)/\text{Serum}\text{IgG}\left(\text{g}/\text{dL}\right)}{\text{CSF}\text{albumin}\left(\text{mg}/\text{dL}\right)/\text{Serum albumin}\left(\text{g}/\text{dL}\right)}$

or

$\text{CSF}\text{IgG}\text{index}=\frac{\text{CSF}\text{IgG}\left(\text{mg}/\text{dL}\right)\times \text{Serum}\text{albumin}\left(\text{g}/\text{dL}\right)}{\text{Serum IgG}\left(\text{g}/\text{dL}\right)\times \text{CSF}\text{albumin}\left(\text{mg}/\text{dL}\right)}$

The reference range for the IgG index varies, reflecting variations in determination of the four index components. A reasonable normal upper limit is 0.8 ( ). However, each laboratory should determine its own critical ratio.

The IgG synthesis rate is calculated by an empirical formula ( ):

$\begin{array}{c}\text{IgG}\text{synthesis}\text{rate}\left(\text{mg}/\text{day}\right)=\left[\right(\text{CSF}\text{IgG}-\text{Serum}\text{IgG}/369)\\ -\left(\text{CSF}\text{albumin}-\text{Serum}\text{albumin}/230\right)\times \left(\text{Serum}\text{IgG}/\text{Serum}\text{albumin}\right)\\ \times 0.43\times 5\text{dL}/\text{day}]\end{array}$

All protein concentrations are expressed in milligrams per deciliter. The first bracketed term represents the difference between measured CSF IgG and the IgG expected from diffusion across a normal BBB; 369 is the normal serum/CSF ratio. The second bracketed term represents the difference between measured CSF albumin and expected albumin if the blood-brain barrier is intact; 230 is the normal serum/CSF albumin ratio. The CSF albumin excess is multiplied by the IgG/albumin ratio and the molecular weight ratio of IgG to albumin (0.43) to correct for changes in CSF IgG due to increased barrier permeability. The number 5 converts the result from a concentration to a daily amount, assuming an average daily CSF production of 500 mL (i.e., 5 dL). The formula does not consider variations in CSF production or Ig consumption. It assumes that the IgG/albumin ratio remains constant over various degrees of BBB impairment—a concept that may lead to variable error ( ). The reference interval for the synthesis rate is –9.9 to +3.3 mg/day. Values greater than 8.0 mg/day indicate an increased rate ( ).

CSF IgG is normally 3% to 5% of total CSF protein. However, in multiple sclerosis (MS), the concentration approaches that of plasma (15%–18%) ( ). The CSF IgG index and the IgG synthesis rate have a sensitivity of 90% in patients with definite MS, but the sensitivity is lower in patients with possible MS, in whom accuracy is most needed ( ). In addition, the specificity for MS is only moderate because increased intrathecal IgG synthesis occurs in many other inflammatory neurologic diseases.

The Ig index and synthesis rate calculations may also be applied to IgM, IgA, Ig light chains, and specific antibodies to infectious microorganisms. For example, increased synthesis of IgM and free κ light chains have been suggested as markers for MS ( ; ).

Electrophoretic Techniques

Although the diagnosis of MS is ultimately a clinical one, significant advances have been made in laboratory testing for this disorder. CSF total protein is increased in less than 50% of patients with MS. Indeed, if the CSF protein exceeds 100 mg/dL, the patient probably does not have MS. However, the γ-globulin fraction, as determined by CSF electrophoresis, is often increased in MS. Thus, the CSF total protein/γ-globulin ratio exceeds 0.12 in about 65% of cases ( ). Using electroimmunodiffusion , a CSF IgG/albumin ratio greater than 0.25 is present in about 75% of cases ( ). Levels greater than the mean CSF IgG index + 3SD are present in 80% to 85% of MS cases, although the upper reference level varies significantly among laboratories.

High-resolution agarose gel electrophoresis of concentrated CSF from patients with MS often shows discrete populations of IgG, the oligoclonal bands . Although these discrete IgG populations are normally absent, two or more bands are necessary to support the diagnosis of MS; a single band is not considered a positive result. Using this technique, oligoclonal bands have been reported in 83% to 94% of patients with definite MS, 40% to 60% of those with probable MS, and 20% to 30% of possible MS cases. However, they are also frequently present in patients with subacute sclerosing panencephalitis, various viral CNS infections, neurosyphilis, neuroborreliosis, cryptococcal meningitis, Guillain-Barré syndrome, transverse myelitis, meningeal carcinomatosis, glioblastoma multiforme, Burkitt lymphoma, chronic relapsing polyneuropathy, Behçet disease, cysticercosis, and trypanosomiasis, among others ( ; ; ; ). Subsequent studies have shown that agarose gel electrophoresis sensitivity for MS is less than previously reported (see later discussion).

Oligoclonal light chains (both κ and λ) are present in about 90% of MS patients ( ; ). They have also occasionally been identified in the CSF of those who are negative for IgG oligoclonal bands. Detection of free light chains in CSF and comparison to serum has been shown to be a sensitive marker for intrathecal immunoglobulin synthesis ( ).

Coomassie brilliant blue or paragon violet stains can resolve oligoclonal bands in only 5 μg of IgG ( ). However, silver staining is 20 to 50 times more sensitive than CBB and can be used on unconcentrated CSF. It is important to note that these electrophoretic techniques must be simultaneously carried out on the patient’s serum to be certain that a polyclonal gammopathy is not present (e.g., liver disease, systemic lupus, rheumatoid arthritis [RA], chronic granulomatous disease) because these disorders may be accompanied by Ig diffusion into the CSF, yielding false-positive results.

Immunofixation electrophoresis (IFE) is more sensitive than agarose gel electrophoresis and does not require CSF concentration ( ). Another study reported a sensitivity of 74% using this technique compared with 57% for agarose gel electrophoresis ( ). More recently, using a semiautomated immunofixation-peroxidase technique, the sensitivity was 83% and the specificity was 79% in patients with clinically definite MS ( ).

Isoelectric focusing and IgG immunoblotting (IgG-IEF) performed on paired CSF and serum samples is the most sensitive and now the recommended method for the detection of oligoclonal bands ( ; ; ). One study showed that IgG-IEF detected 100% of definite MS, but only about 50% were positive by agarose electrophoresis ( ). Others detected 91% of MS cases but only 68% with agarose ( ). Similarly, a semiautomated IgG-IEF technique identified 90% of MS cases compared with 60% for agarose electrophoresis ( ). In 2005, an international consensus standard for the diagnosis of MS established IgG-IEF as the method of choice for qualitative detection of oligoclonal IgG bands as evidence of intrathecal synthesis of IgG ( ). More recent studies have reinforced the utility of IgG-IEF in diagnosing MS and distinguishing this disease from other disorders ( ). Figure 30.9 shows the different patterns seen on IgG-IEF in paired CSF and serum samples from patients with MS and other conditions.

Oligoclonal bands (OCBs) in CSF have a high sensitivity but low specificity for MS; thus, they are useful for screening but not for diagnosis. The overlap of positive OCB findings in infectious and other inflammatory conditions has stimulated the search for new biomarkers in CSF that are more specific for MS (Gastaldi et al., 2017) and that have potential prognostic relevance. Candidate markers showing promise include the following:

• Immunoglobulin free light chains (FLCs) as an indicator of B-cell activity are found in serum as well as other body fluids and could potentially be useful for diagnosing true MS patients who happen to be OCB negative. Early studies indicate utility for FLC measurements in CSF (especially because such measurements are automated); however, larger patient groups should be evaluated for wide acceptance ( ).

• Oligoclonal IgM bands in CSF appear to be a very powerful marker for more severe and more aggressive MS ( ). Unfortunately, hydrophobicity of IgM requires its chemical reduction prior to electrophoresis, which may still be affected by artifacts from reassociation of monomeric IgM. Nevertheless, oligoclonal IgM in CSF (which is directed against myelin lipid, usually phosphatidylcholine) is considered a very promising prognostic test for MS but will require more developmental work for standardization ( ).

• The measles-rubella-zoster (MRZ) reaction consists of measuring antibody titers to those viruses in CSF and serum. The MRZ reaction is positive if at least two of those indexes (CSF titer/serum titer) are positive. MRZ is considered highly specific for MS and could be recommended for patients with suspected MS who are OCB negative ( ).

• Neurofilaments are released into CSF following neuron and axon injury. They are also found in serum and, as such, could be used to monitor progression of MS as a highly prognostic marker once measurements become well established and are widely practiced ( ).

Further advances in CSF proteomic testing whereby literally hundreds of individual protein species can be identified is likely to provide more diagnostic power for neurologic diseases in coming years ( ; ).

In summary, the diagnosis of MS, as with many other neurologic disorders, is ultimately a clinical one based on neurologic history and physical examination. Nevertheless, advanced laboratory results in CSF—such as elevated IgG indices, particularly the detection of oligoclonal IgG bands and neuroimaging techniques—have proved invaluable in the diagnosis of MS.

Glucose

Derived from blood glucose, fasting CSF glucose levels are normally 50 to 80 mg/dL (2.8–4.4 mmol/L)—about 60% of plasma values. Rigorous evaluation of CSF and serum glucose levels in a large number of children confirmed that the CSF glucose level is normally approximately 60% of the serum glucose level during childhood ( ). Results should be compared with plasma levels, ideally following a 4-hour fast, for adequate clinical interpretation. The normal CSF/plasma glucose ratio varies from 0.3 to 0.9, with fluctuations in blood levels caused by the lag in CSF glucose equilibration time. A good rule of thumb is that CSF glucose is about two-thirds of the level of serum glucose. This enables the detection of low CSF glucose in patients with abnormal glucose levels. Thus in a diabetic patient with a serum glucose level of 250 mg/dL, the expected CSF glucose would be around 167 mg/dL in CSF. If the patient were found to have a CSF glucose of, say, 100 mg/dL, there would be evidence for a possible microorganism-induced lowering of CSF glucose even though the absolute value of CSF glucose is elevated.

CSF values below 40 mg/dL (2.2 mmol/L) or ratios below 0.3 are considered to be abnormal. Hypoglycorrhachia is a characteristic finding of bacterial, tuberculous, and fungal meningitis. However, sensitivity can be as low as 55% for bacterial meningitis ( ); thus, a normal level does not exclude these conditions. Some cases of viral meningoencephalitis also have low glucose levels but generally not to the degree seen in bacterial meningitis. Meningeal involvement by a malignant tumor, sarcoidosis, cysticercosis, trichinosis, ameba ( Naegleria ), acute syphilitic meningitis, intrathecal administration of radioiodinated serum albumin, subarachnoid hemorrhage, symptomatic hypoglycemia, and rheumatoid meningitis may also produce low CSF glucose levels ( ).

Decreased CSF glucose results from increased anaerobic glycolysis in brain tissue and leukocytes and from impaired transport into the CSF. Bacteria are usually present in insufficient quantities to be a major contributor to the use of glucose in CSF. CSF glucose levels normalize before protein levels and cell counts during recovery from meningitis, making it a useful parameter in assessing response to treatment.

Increased CSF glucose is of no clinical significance, reflecting increased blood glucose levels within 2 hours of lumbar puncture. A traumatic tap may also cause a spurious increase in CSF glucose.

Lactate

CSF and blood lactate levels are largely independent of each other. The reference interval for older children and adults is 9.0 to 26 mg/dL (1.0–2.9 mmol/L) ( ). Newborns have higher levels, ranging from about 10 to 60 mg/dL (1.1–6.7 mmol/L) for the first 2 days and from 10 to 40 mg/dL (1.1–4.4 mmol/L) for days 3 to 10 ( ). Elevated CSF lactate levels reflect CNS anaerobic metabolism due to tissue hypoxia.

Lactate measurement has been used as an adjunctive test in differentiating viral meningitis from bacterial, partially treated bacterial, mycoplasma, fungal, and tuberculous meningitis, in which routine parameters yield equivocal results ( ). In patients with viral meningitis, lactate levels are usually below 25 mg/dL (2.8 mmol/L) and are almost always less than 35 mg/dL (3.9 mmol/L), whereas bacterial meningitis typically has levels above 35 mg/dL ( ; ). Using 30 to 36 mg/dL as the cutoff value for bacterial meningitis, the sensitivity and specificity are about 80% and 90%, respectively. Viral meningitis, partially treated bacterial meningitis, and tuberculous meningitis often have intermediate lactate levels that overlap each other, limiting the use of lactate measurements in this differential diagnosis.

Persistently elevated ventricular CSF lactate levels are associated with a poor prognosis in patients with severe head injury ( ).

F2-Isoprostanes

Isoprostanes are prostaglandin-like compounds resulting from non-enzymatic free radical-initiated peroxidation of arachidonic acid. A subclass of these compounds have a characteristic so-called F2 ring structure and are therefore called F2-isoprostanes. F2-isoprostanes are increased in diseased regions of the brain in patients with AD ( ) and are markers of free radical brain injury associated with advanced age and latent AD ( ). Compared with age-matched controls, CSF F2-isoprostanes are elevated in patients with probable AD ( ). Therefore, in conjunction with CSF τ and β-amyloid protein, the measurement of CSF F2-isoprostanes appears to enhance the accuracy of the laboratory diagnosis of AD ( ) and by itself appears to correlate with the degree of cognitive decline in some AD patients ( ).

Enzymes

A wide variety of enzymes derived from brain tissue, blood, or cellular elements have been described in the CSF. Although CSF enzyme assays are not commonly used in the diagnosis of CNS diseases, there are diseases/disorders in which they may prove useful.

Ammonia, Amines, and Amino Acids

CSF ammonia levels vary from 30% to 50% of blood values. Elevated levels are generally proportional to the degree of existing hepatic encephalopathy but are difficult to quantify. Moreover, because hepatic encephalopathy generally correlates with blood ammonia levels, the measurement of CSF ammonia has little, if any, clinical value. However, cerebral glutamine, synthesized from ammonia and glutamic acid, serves as the means for CNS ammonia removal. Thus, CSF glutamine levels reflect the concentration of brain ammonia. Glutamine reference intervals are method dependent; the upper reference level is about 20 mg/dL. Values over 35 mg/dL are usually associated with hepatic encephalopathy ( ). Elevated CSF glutamine levels have also been reported in patients with depression ( ) and encephalopathy secondary to hypercapnia and sepsis ( ). Measurement of a variety of biogenic amines in CSF has been shown to assist in the diagnosis of bacterial and viral meningitis ( ).

A major etiologic theory of schizophrenia involves dopamine. The cornerstone for this theory is the fact that neuroleptic drugs that block dopamine receptors are effective in the treatment of this disorder. Thus, it has been reported that CSF levels of homovanillic acid (HVA), a metabolite of the biogenic amines, are related to the severity of schizophrenic psychosis ( ). However, HVA concentration varied as a function of psychosis rather than being related to the diagnosis of schizophrenia per se. Others have reported decreased CSF levels of 5-hydroxyindoleacetic acid, a metabolite of serotonin, in schizophrenic patients with suicidal behavior ( ). This report adds support for a possible relationship between suicide and CNS serotonin metabolism.

Although free CSF amino acids are relatively high in infants younger than 30 days of age, their concentration is further increased in those with febrile convulsions and bacterial meningitis. γ-Aminobutyric acid (GABA), a major inhibitory brain transmitter, is significantly decreased in basal ganglia neurons and is very low or undetectable in the CSF of patients with AD and Huntington disease ( ; ). CSF GABA was detected in all patients with migraine attacks, but not in those with tension headaches or in the control group without headaches ( ). Infants with startle disease, a rare inherited autosomal-dominant disorder characterized by seizures or the so-called stiff baby syndrome , have significantly decreased CSF GABA levels ( ; ).

Electrolytes and Acid-Base Balance

There are no clinically useful indications for the measurement of CSF sodium, potassium, chloride, calcium, or magnesium. Measurements of CSF pH, partial pressure of carbon dioxide (pCO ₂ ), and bicarbonate are also not practical for patient care ( ).