A study of atypical APTT derivative curves on the ACL TOP coagulation analyser

INTRODUCTION: The graphical representation of clotting data can provide useful information. A novel feature of the ACL TOP software allows display of clot reaction curves with superimposed first and second derivative curves. We noted atypical derivative plots associated with normal 'S'-shaped clot reaction curves in some abnormal activated partial thromboplastin time (APTT). METHODS: We tested 179 citrate plasmas with four different APTT reagents on the ACL TOP. We documented the prevalence and utility of these APTT atypical derivative curves in these patients. RESULTS: Ellagic acid APTT reagents commonly produce atypical derivative curves despite normal 'S'-shaped clot reaction curves. Occasionally, these atypical second derivative curves may cause incorrect APTT results. With silica activator APTT reagents, atypical derivative curves were associated with genuine coagulation abnormalities such as single factor deficiencies and lupus anticoagulants (LAs). To our knowledge, this novel finding has not been reported. CONCLUSION: Atypical derivative curves seen in ellagic acid APTTs are of limited diagnostic use because of the frequency with which they occur. This may be related to the need to optimize the data reduction utilized on the ACL TOP for these reagents. With silica activator APTTs, the presence of atypical derivative curves proved to be a very simple tool when troubleshooting unexpected abnormal APTT results, commonly predicting a factor deficiency or LA that would warrant further investigation. The cause of these aberrant derivative curves is probably related to abnormal thrombin generation in the APTT test and warrants further study.