Our August 2022 Quick Question attracted 34 responses [despite our discouraging “Recaptcha” security function] and asked, “You test for lupus anticoagulant [LAC] using a low-PL PTT reagent and a hex-phase high-PL mix. The mix results are 10 seconds shorter than the first test. What do you conclude?”
- LAC positive 32% 
- LAC negative 18% 
- Inconclusive 15% [5\
- First confirm with DRVVT 35% 
Although the value is subject to local validation, in most instances using either photo-optical or electromechanical [viscosimetric] instrumentation, when the high-phospholipid reagent shortens the PTT by 8 seconds or more, the conclusion is that LAC is present, on the principle that the reagent phospholipid partially neutralizes the LAC. Those who chose “LAC negative” may have been expecting a more pronounced shortening effect based on local validation. Those who chose “Inconclusive” may have been looking for the “raw” PTT results before reaching their decision.
One noteworthy outcome of this month’s Quick Question is that apparently many of us conclude incorrectly that the PTT-based results must be confirmed by DRVVT-based results, or vice-versa. In fact, we use two testing platforms to address LAC heterogeneity, so that a positive result in either the PTT or DRVVT-based assay is interpreted as positive for LAC, provided we rule out the presence of a specific inhibitor such as anti-FVIII or anti-FIX. The two platforms may not confirm each other.
As always, we solicit your comments below.