[PubMed] [Google Scholar]. positive by FLI for anti-FVIII IgG4. Conversely, 91% of NBA-positive samples from haemophilia subjects were positive for anti-FVIII IgG4. Two of 11 haemophilia subjects had samples unfavorable for anti-FVIII IgG4 and CBA, which likely represented LA rather than FVIII inhibitor presence. Conclusions: Assessment of anti-FVIII profiles Dooku1 along with the CBA may be useful to distinguish a clinically relevant low-titre FVIII inhibitor from a transient LA in HA patients. = 0 for the subjects initial study sample. All anti-FVIII IgG4 results were positive for subjects H4 and H6. NBA and CBA titres varied, giving positive and negative readings at different timepointsbut neither subject was successfully tolerized during the study or 1-year follow-up period. Subject H8 initially had a negative anti-FVIII IgG4, which became positive during the course ITI of ITI therapy. At the final timepoint, subject H8 was considered tolerized, which was reflected in his unfavorable anti-FVIII IgG4, NBA and CBA results and he has remained tolerized during the 1-y post-study follow-up period 4 ?DISCUSSION To determine appropriate management for patients with inhibitors, clinicians rely on composite assessment of clinical history, bleeding manifestations and inhibitor titre as measured by functional assays; however, LAs are known to cause false-positive results in the NBA.12,13 Dooku1 This problem is exacerbated by the lack of a definitive diagnostic test Dooku1 for LAs and by documented inconsistencies in laboratory testing for FVIII inhibitors.19,20 The CBA is less influenced by a LA than the NBA or BA; and anti-FVIII IgG1 and anti-FVIII IgG4 subtypes have been shown to correlate better with detection of a neutralizing haemophilic inhibitor by CBA than by NBA.5,13 The current study aimed to improve understanding of the effect of a LA on different FVIII inhibitor assays. Our hypothesis was that the immunoreactive profile generated by anti-FVIII FLI could be used to distinguish a haemophilia patient with a LA from a haemophilia patient with a clinically relevant FVIII inhibitor. The discriminatory value of the anti-FVIII IgG4 assay for distinguishing LAs from FVIII inhibitors is usually supported by our data which show that none of the 41 samples from the non-haemophilia study group Dooku1 (with positive LA assessments)including those with either a positive NBA or CBA titrewere positive in the anti-FVIII IgG4 assay. These results are similar to those observed in healthy subjects by Whelan et al7 using an ELISA and by Boylan et al using the FLI. The unfavorable results in the FLI strongly support our hypothesis that this assay is usually unaffected by the presence of Las, whereas the commercial anti-FVIII ELISA, which is usually reported to measure IgG but is not specific for IgG4, has been reported to give positive results in some LA patients.21 Our results show IgG subclasses other than IgG4 may be present in LA patients and IL6R could influence results of this ELISA test. Our data suggest that a specific anti-FVIII IgG4 assay is able to discern a LA from a low-titre FVIII inhibitor with a high discriminatory value, while anti-FVIII IgG1, IgG2, IgG3 and IgGM are less useful. The observation that 12 (29%) LA-positive samples from the non-haemophilia study group were positive in the NBA and 2 (5%) in the CBA in the absence of anti-FVIII IgG4 confirm that these functional assays may be subject to interference. Data from.