Abstract
The International Society on Thrombosis and Haemostatis (ISTH) has established criteria for the diagnosis of lupus anticoagulants (LA) to include a mixing study to establish the presence of an inhibitor. Up to 30% of LAs are time and temperature dependent inhibitors, therefore only an incubated mixing study will detect this subtype. Global assays (i.e. STACLOT-LA® Diagnostica Stago) incorporate a sensitive screening reagent, a source of normal pooled plasma (npp) for an immediate mixing study, and a step to demonstrate phospholipids dependence. It does not, however, include an incubated mixing study. 493 consecutive STACLOT-LA samples were evaluated based on outcome and compared to the clotting time for Tube 1 (sample + npp + buffer) and Tube 2 (sample + npp + buffer + hexagonal phospholipids). All samples were tested according to manufacture’s instructions with a delta > 8 seconds considered positive. Figure 1 reveals the lack of clear demarcation between negative and positive outcomes. Figure 2 identifies considerable differences between the clotting times of tube 1. Tube 2, while statistically different, has greatly overlapping confidence intervals. From this, it was identified that samples with Tube 1 clotting times between 55–75 seconds represented the population most likely to give borderline confirmatory calculations (CC). Subsequently, 45 samples with this criterion were selected for study, 17 negatives and 28 positives (CC 6.2–9.7). Immediate and 1 hour incubated mixing studies were performed with two reagents - aPTT-A and aPTT-LA® (Diagnostica Stago). The results are given in the table.
Scatter plot of LA Calculation
| aPTT-A | aPTT-LA | |||||
|---|---|---|---|---|---|---|
| Correction Reference Interval | NC Reference Interval | NC npp + 5 seconds | Correction Reference Interval | NC Reference Interval | NC npp + 5 seconds | |
| NC= No correction; npp= Normal pooled plasma time; †2 of 3 LA positive by dRVVT; *1 case is a time dependent inhibitor | ||||||
| Positive, (n=28) | 22 (78) | 6 (21) | 13 (46) | 23* (82) | 5 (18) | 14 (50) |
| Negative, (n=17) | 14 (82) | 0 | 3† (18) | 14 (82) | 0 | 3† (18) |
| aPTT-A | aPTT-LA | |||||
|---|---|---|---|---|---|---|
| Correction Reference Interval | NC Reference Interval | NC npp + 5 seconds | Correction Reference Interval | NC Reference Interval | NC npp + 5 seconds | |
| NC= No correction; npp= Normal pooled plasma time; †2 of 3 LA positive by dRVVT; *1 case is a time dependent inhibitor | ||||||
| Positive, (n=28) | 22 (78) | 6 (21) | 13 (46) | 23* (82) | 5 (18) | 14 (50) |
| Negative, (n=17) | 14 (82) | 0 | 3† (18) | 14 (82) | 0 | 3† (18) |
Conclusions: Borderline positive STACLOT-LA results showed an inhibitor pattern only 50% of the time under the most optimal conditions of a LA sensitive aPTT reagent with select interpretative guidelines. Therefore, a borderline or indeterminate range might be a more effective way to communicate these results if the inclination is to avoid a separate mixing study.
Disclosure: No relevant conflicts of interest to declare.
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