aPS/PT antibodies: specificity and sensitivity compared to other aPL assays
As a first step to set in our population the cut-off of the new tests for aPS/PT IgG and IgM antibodies, we performed the analysis in a series of 52 healthy donors. The cut-off suggested by the manufacturer was ≥30 U/ml both for IgG and IgM. Considering this value, the specificities were, respectively, 98.1 % for aPS/PT IgM and 86.5 % for aPS/PT IgG. Setting the cut-off of IgG at 40 U/ml, the specificity improved (94.2 %), thus we finally decided to consider frankly positive aPS/PT IgG when ≥40 U/ml, borderline between 30 and 40 U/ml.
Overall, aPS/PT IgG and/or IgM positive antibodies were found in 49/421 (11.6 %) cases, comprising 37 (75.5 %) IgM positive, 11 (22.4 %) IgG frankly positive (≥40 U/ml) patients and one aPS/PT IgM/IgG double positive case. Borderline aPS/PT IgG (30-40 U/ml) antibodies were present in 16 cases (two of which were also positive for aPS/PT IgM). After adding these borderline IgG cases, the ultimate aPS/PT sensitivity was assessed at 15 % (63/421).
aCL antibodies were simultaneously analyzed in 403 of the 421 cases, resulting in 4 (1 %) IgG positive, 20 (4.9 %) IgM positive and 5 (1.2 %) IgG/IgM double positive cases.
The aβ2GPI antibodies were simultaneously analyzed in 398 of the 421 patients, resulting in 25/398 (6.3 %) positive cases: 4 IgG, 19 IgM and 2 IgG/IgM double positive cases.
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Data about LA were available in 367 of the overall 421 cases, of which 41/367 (11.2 %) resulted positive. Among the LA positive patients, 23/41 (56.1 %) resulted aPS/PT positive: of note all these cases were IgM positive, while only one was also IgG frankly positive. In contrast, in the same subgroup of LA positive subjects, only 12/41 (31.7 %) resulted aCL (p = 0.023, 95 % CI 1.24-7.69, OR 3.09) and 11/41 (26.8 %) aβ2GPI positive (p = 0.013, 95 % CI 1.38-8.80, OR 3.49).
In general, the simultaneous presence of aPS/PT and aCL or aβ2GPI antibodies was infrequent; in particular, only one of the aPS/PT IgG frankly positive patients (1/12, 8.3 %) resulted also aCL and/or aβ2GPI positive, while among the aPS/PT IgM positive patients, 9/38 (23.7 %) were also aCL and/or aβ2GPI positive.
Further important observations came from the analysis of LA positive and negative patients.
In LA positive patients, testing of aPS/PT antibodies allowed to identify 39 % previously unrecognized aPL antigenic specificities, while the simultaneous presence of the aPS/PT and the aCL and/or aβ2GPI antibodies was noticed in 17.1 % of cases, the aCL plus the aβ2GPI antibodies in 9.7 % and the sole aCL antibodies in 4.9 % (Fig. 1).
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The analysis of the 310 LA negative cases in which all the aPL antibodies were tested (Fig. 2), revealed 6.1 % aPS/PT positive patients (equally distributed between IgG and IgM) and 3.9 % aCL plus aβ2GPI antibodies positive cases (the large majority IgM).
aPS/PT antibodies testing helps to interpret uncertain LA test results
Patients referred by the Haemostasis Unit to improve the interpretation of LA tests were eventually classified as 26 LA positive, 19 LA negative and 17 inconclusive because of OAT. The majority (50/62, 80.6 %) of these patients presented repetitively aCL and aβ2GPI negative antibodies in their follow-up. Our attention was initially focused on sera that were negative for aCL and aβ2GPI antibodies and positive for LA (no. 21): in this subgroup, we disclosed 10/21 (47.6 %) aPS/PT positive cases (4 IgG and 6 IgM), (Table 1).
Important results came also from the analysis of the subgroup of patients in OAT (Table 1): 5/17 (29.4 %) patients showed positive aPS/PT IgG or IgM antibodies, while 4/17 (23.5 %) were aCL and/or aβ2GPI positive (two of which also aPS/PT positive). Therefore, testing for aPS/PT antibodies allowed us to find out 3/17 (17.6 %) previously unrecognized aPL positive subjects.
Finally, among the LA negative patients (Table 1), we discovered 4/19 (21.1 %) aPS/PT positive, only two aCL positive and one aPS/PT plus aCL positive cases.
Comparison between aPS/PT and aPT antibodies in LA-positive patients
We retrospectively analyzed the prevalence of the aPS/PT antibodies compared to the prevalence of the aPT antibodies in a series of 52 LA-positive patients previously investigated for aPT antibodies. As illustrated in Table 2, aPT IgG and/or IgM antibodies were positive in 8 patients (15.4 %), while the aPS/PT IgG and/or IgM antibodies in 29 (55.8 %; p < 0.0001; 95 % CI 2.73-17.60, OR 6.94). Both aPS/PT and aPT antibodies were positive in 5 cases (9.6 %), while aPT alone in 3 (5.8 %) and aPS/PT alone in 24 (46.1 %).
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