Here is a provocative follow-up from Dr. Ning Tang describing PTT reagent sensitivity to LAC:
Hi, Prof George,
We performed an experiment to compare APTT of samples with or without LAC by different APTT reagents. The results suggest that the type of activator in APTT reagent seems not to be the determinant of sensitivity for LAC (see in attachment), I hope that’s interesting for our discussion.
Attachment: /sites/default/files/book1.xls.pdf
Hello, Dr. Ning Tang, the cases you’ve tested seem to indicate near-equal sensitivity by silica-based and ellagic acid-based reagents, confirming the comments from Mr. McGlasson and Dr. Favaloro. Although at first glance the silica-based PTT reagents appear to have misclassified the weak LAC, in all three cases the results are borderline when compared to the upper limit of the RI.
From Sheila A. Minor: Dear
From Sheila A. Minor: Dear George, It seems that the sensitivity to LAC depends on the phospholipid composition rather than the type of activator in the APTT reagents. Is it advisable to screen with a moderately sensitive APTT reagent such as Stago’s PTT
Automate, but if a LAC profile is ordered for further investigation, should a highly sensitive APTT reagent such as Stago’s PTT–LA reagent be used to repeat the patient’s PTT? Would this be in compliance with the ISTH recommendations? We currently include in our LAC profile the following tests: Precision Biologic’s DRVVT Screen, CryoChek LA Check; and if abnormal, confirm and ratio and 50:50 Mix DRVVT Screen. We also use the Staclot LA hexagonal phase test. Also, do you recommend using the Rosner Index to help with the interpretation of a PT/PTT mixing study results? Thank you!
Hi, Sheila, thank you for your question. Your approach is correct, it is appropriate to use an intermediate sensitivity PTT reagent for general screening and heparin management and a LAC-sensitive reagent like PTT–LA for LAC detection. You confirm using the hex-phase StaClot kit and the DRVVT confirmatory reagent, CryoChek LA Sure. The Rosner index is reliable for establishing correction, as it bases the decision on the normal plasma and the patient plasma values. The usual decision point is a ratio of 11. We use a simpler approach, establishing the limit as 10% above the normal plasma value. Thank you for your response!