Coagulation: Specimen Integrity, Mixing Studies, and Inhibitors
The coagulation section tests specimen integrity, interpretation of screening and mixing studies, factor assays, and inhibitor workups.
Specimen integrity
Citrate tubes contain 3.2% sodium citrate and must be filled to the 9:1 blood-to-anticoagulant ratio; short draws artificially prolong clotting times because excess citrate binds calcium. Patients with hematocrit above roughly 55% need the citrate volume reduced. Test plasma within about four hours at room temperature, and rule out heparin contamination from a line draw before believing a prolonged aPTT.
Screening tests
The PT/INR monitors warfarin and reflects factors VII, X, V, II, and fibrinogen; the INR uses the ISI and the mean normal PT. The aPTT reflects the intrinsic and common pathways and is the traditional heparin monitor, though anti-Xa is often preferred. In mixing studies, correction (often defined as at least 50% of normal factor activity) indicates factor deficiency, while failure to correct at immediate and 2-hour incubation suggests a specific inhibitor or a lupus anticoagulant.
Lupus anticoagulant
Screen with a phospholipid-dependent test (dRVVT or a sensitive aPTT), then confirm phospholipid dependence using the hexagonal phase or platelet neutralization procedure. Guidelines require testing on two occasions at least 12 weeks apart, because transient positivity follows infection and thrombosis.
Worked example
An aPTT is 74 s (reference 25–35). The mixing study is 72 s immediately and 70 s after incubation - no correction, so an inhibitor is present. The PT is normal, and the dRVVT ratio is 2.1, correcting with excess phospholipid. The diagnosis is lupus anticoagulant, not factor deficiency, and the patient is paradoxically prothrombotic.
Traps that cost marks
Ordering factor assays before a mixing study. Interpreting a prolonged aPTT as heparin when the specimen came from an EDTA tube. Overlooking DOAC interference (notably rivaroxaban and apixaban) in dRVVT-based lupus anticoagulant testing.