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Mixing Studies
In mixing studies, patient plasma with a prolonged activated partial thromboplastin time (PTT) or prothrombin time (PT) is mixed with normal pool plasma (NPP). PTT or PT is measured after mixing the two samples. Mixing studies are used to distinguish among potential causes for prolonged screening test results—in particular, to distinguish between a factor deficiency and the presence of an inhibitor.
Methods
The standard procedure for PTT or PT is performed on a 1:1 mixture of patient plasma and NPP, usually immediately and after 1 hour incubation at 37°C.
Interpretation
Mixing study outcomes are summarized in Table 130.1 . Complete correction of a 1:1 mix suggests a factor deficiency, either congenital or acquired. Failure to correct completely suggests an inhibitor interfering with one or more coagulation factors or a lupus anticoagulant. If repeating the PTT mix after incubation for 1 hour at 37°C gives a longer time, a FVIII inhibitor, which is time-dependent, may be present. Rarely, time-dependent inhibitors may be directed against FV. A PT mix that corrects most often indicates a multifactor deficiency due to the presence of liver disease, vitamin K deficiency, or warfarin; however, it may represent FVII deficiency alone.
Table 130.1
Interpretation of Activated Partial Thromboplastin Time (PTT) and Prothrombin Time (PT) Mixing Studies in Specimens Without Heparin
| Observation | Possible Cause | Tests to Perform |
|---|---|---|
| PTT 1:1 Mix of Patient and NPP a | ||
| Corrected immediately and at 1 hour | Factor deficiency | Factor VIII, IX, XI, XII activities (factors II, V, X if PT also prolonged) |
| Corrected immediately and prolonged at 1 hour | Factor VIII inhibitor | Factor VIII activity; if low, FVIII inhibitor |
| Weak LA b | LA tests | |
| Incompletely corrected | LA | LA tests |
| Factor inhibitor | Factor VIII, IX, XI, XII activities | |
| PT 1:1 Mix of Patient and NPP | ||
| Corrected immediately | Factor deficiency | Factor II, V, VII, X activities |
| Incompletely corrected | LA | LA tests |
| Factor inhibitor | Factor II, V, VII, X activities | |
The definition of “correction” varies from laboratory to laboratory because of differences in the PTT and PT methods used. Some thromboplastin reagents are more sensitive than others. Attempts to standardize across laboratories have been generally unsuccessful. Consultation with the particular laboratory involved or someone experienced in interpreting its results may be helpful, particularly when partial correction occurs.
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