Background. The presence of pre-transplant HLA donor specific antibodies (DSA) is a recognized risk factor for acute rejection. However, due to the test's (LUMINEX) high specificity, it is not possible to determine which DSA(s) are pathogenic. The correlation between the DSA mean fluorescence intensity (MFI) and the FCxM result have been demonstrated in different series. The C1q assay may be an alternative to distinguish complement-fixing (pathogenic) vs. non-complement-fixing HLA antibodies. Aim: To establish the DSA MFI level with the best sensitivity and specificity in predicting FCxM and compare it with the C1q assay's predictive capacity in patients with DSA(s) and AHG-CDC CxM negative. Methods. We used cryopreserved (-70°C) sera from potential renal transplant recipients (PRT), containing DSAs against their respective donors – all had negative AHG-CDC CxM and either positive or negative FCxM. A MFI ROC curve was plotted and the point of best concordance with FCxM was determined. Sera were evaluated with the LUMINEX C1q assay. The sensitivity and specificity of the C1q assay were determined as well as that of the DSA with a pre-determined MFI for FCxM. Results: Fifty-eight PRT were evaluated, 5 of which had 2 potential donors. We analyzed 63 tests of which 33 (52.3%) had a positive FCxM. The average MFI of the dominant DSA was 6,337 MFI. A MFI of 2300 performed best.

When joining the C1q assays with the DSA >2300 MFI, sensitivity improved but specificity decreased.(Sens. 96.97% (CI 89.6-100) Spec. 60% (40 -79%) PPV 72.7% (58 – 87) NPV 93.33% (82.06 – 100). Conclusions: The C1q assay is a better predictor of the FCxM result when compared with only the presence of DSA. DSA values with a MFI <2300 and not binding C1q, can be considered to yield a negative FCxM.

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