From Kirk Guyer, Cascade Technologies: There are whole blood PTT (APTT ) assays and there are citrated platelet poor plasma (PPP ) PTT assays. What happens in a PPP PTT assay if there are platelets present? Does the PTT result shorten? Can platelets shorten the PTT result in a whole blood PTT assay? I ran a PTT on a citrated whole blood sample and it gave a normal PTT. When I centrifuged the sample and ran the citrated plasma it gave an abnormal value which matched the reference PTT method. What might be causing the false negative in the whole blood sample? The hematocrit was normal.
George's answer to Kirk: routine PTTs performed as screens to detect coagulopathies may be performed on plasma prepared by customary centrifugation and don't necessarily require PPP (plasma whose platelet count is less than 10,000/uL). However, most lab scientists prepare PPP for their routine PTTs and PTs as a matter of convenience, since PPP is required for lupus anticoagulant testing and coagulation factor assays. Also, when using the PTT to assay heparin, platelets secrete heparin-neutralizing platelet factor 4, so PPP is necessary if there is to be a delay of over one hour from collection to testing. Accriva's Hemochron is the only instrument that offers a whole blood PTT , but this is performed as a POC assay, so specimen storage and platelet secretions are not an issue.
However, I'm assuming you ran both sets of assays on "in-laboratory" electro-mechanical instrumentation, given that whole blood would not work in an optical system. In this instance, I speculate that whole blood, compared to PPP , offers additional coagulation factor and fibrin assembly points; the RBC membranes, and perhaps even more so, WBCs, since WBC membranes are physiologically active. This would speed up the fibrin formation and account for the factitiously shortened PTT. It may be that whole blood coagulation testing more closely approximates in vivo hemostasis.