Measurement of Platelet Count, Mean Platelet Volume, and Reticulated Platelets

Numerical platelet determination is a critical step in assessing bleeding and thrombosis risk. Additionally, platelet count serves as a biomarker for a number of pathologic states. Modern automated hematology analyzers produce fast, accurate, and precise platelet determination under most circumstances. Accuracy of automated platelet counts may be compromised by platelet clumping, platelets of unusual size, small red blood cells (RBC), or cell fragments. Depending on methodology, size and/or light scatter properties are commonly used. Very low platelet counts can also pose a challenge due to problems in distinguishing small platelets from background noise. Manual platelet counts can alleviate some of the problems inherent in the automated counts but may be imprecise and labor-intensive. The current gold standard method is based on a combination of light-scatter gating and staining with antibodies directed against platelet-specific antigens.

Platelets are central to proper hemostatic function. They participate both in primary hemostasis through formation of the temporary plug after injury and in secondary hemostasis by providing the phospholipid surface needed for clot formation and secreting factors that promote the clotting cascade.

Platelets, at 2–3 μm in diameter, are the smallest cellular elements in the plasma. They are produced in the bone marrow through fragmentation of megakaryocyte cytoplasmic projections that invade marrow vascular spaces. Under normal steady-state conditions, roughly 100–200 billion platelets are produced and destroyed per day. Platelet life span is 5–10 days under normal circumstances.

Newly produced platelets are larger in size and contain residual messenger RNA. They are considered to be the platelet analogue of red cell reticulocytes and are termed “reticulated” platelets. Because mRNA degrades within 24 hours in vivo, reticulated platelet counts are a good measure of megakaryocyte synthetic activity.

Increased average platelet size is also a sign of increased platelet turnover and activity and is associated with inflammation. It has also been linked with elevated risk of arterial thrombotic events. Decreased platelet volume may be seen in some congenital platelet disorders and depressed platelet production states. The platelet volume can be captured in the mean platelet volume (MPV) parameter.

Like most hemostatic factors in circulation, platelets are present in excess of what is minimally required to maintain hemostasis under normal circumstances. A healthy adult has roughly 150–450 thousand platelets per microliter of blood. In addition to the platelets present in circulation, the spleen holds approximately one-third of the total number of platelets in the body. Numerous pathologic conditions can reduce platelet numbers in the blood. However, reduction in circulating platelets does not always result in a substantially increased risk of hemorrhage. Recent studies have shown that as few as 5000/μL platelets are adequate to maintain vascular integrity in the absence of significant hemostatic stress. Under minimal stress, such as a minor surgical procedure, 30–50,000/μL platelets appear sufficient. 100,000/μL platelets are desirable for major surgery.

On the other end of the spectrum, increased platelet counts may contribute to the pathogenesis of some hematologic disorders. Abnormally high platelet counts in myeloproliferative disorders correlate with a risk of thrombosis and may signal a more aggressive course of the disease. Moreover, abnormally elevated platelet counts may reduce concentration of functional von Willebrand factor through clearance, leading to acquired von Willebrand disease. This can paradoxically elevate the risk of hemorrhage.

As studies show the transfusion threshold should be as low as 10,000/μL platelets for most thrombocytopenic patients, it is imperative for laboratories to achieve adequate precision at these low platelet counts. This requirement still poses challenges in routine practice.