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    • The possibility of having false positives even

    2022-07-02

    The possibility of having false positives, even if minimized, is always present and requires that a reactive result should be followed by a confirmation test. Traditionally, Western blot (WB) has been used, but various types of immunoblotting that use recombinant PAC-1 or synthetic peptides are also on the market and, regarding the WB, various international criteria of positivity were defined [19]. Given the importance of the 4th generation tests, since the third generation tests are nowadays no more appropriate for clinical HIV diagnostics, more and more companies are developing and commercializing these tests whose performances have been evaluated both by comparing the results among themselves and by 3rd generation tests [8,[20], [21], [22], [23], [24], [25], [26]]. Recently, a new automated 4th generation enhanced chemiluminescence assay VITROS HIV Combo (Ortho-Clinical Diagnostics) was made available on the market. The aim of this work was to compare and evaluate the VITROS HIV Combo assay against a similar 4th generation assay and a 3rd generation assay currently in use at the Microbiology Unit of Hospital of Legnano for the normal laboratory routine.
    Study design
    Results
    Conclusions In an infectious serology laboratory, the search for HIV infection markers represents a significant proportion of routine work. Currently on the market, in addition to third generation tests [13,27,28] there are also fourth generation tests which performances have been compared with each other and with third generation tests [8,[20], [21], [22], [23], [24], [25], [26]]. Data in various studies have shown that despite the differences reported in the various parameters between the 4th generation tests, in general the differences are not statistically significant [13,29,30]. On the other hand, the test formulation can be responsible for the slight differences in the results whose entity must be known. We undertook to evaluate the performance of a 4th generation test recently developed and commercialized by Ortho-Clinical Diagnostics, using routing samples from a microbiology laboratory collected and examined daily from different populations. Three different groups of samples were examined. The second group included samples known to be HIV-positive with different subtypes and viral load. The samples had been frozen for no more than 4 weeks from the collection date, in accordance with the maximum sample conservation limit tested by Ortho. The VITROS HIV Combo test, showing no significant differences in the signal obtained from different HIV subtypes (unlike the other two tests) confirming the result of a previous study on a commercial panel [31]. Similarly, no differences were observed in samples of HIV positive subjects receiving antiretroviral therapy and with negative or positive viraemia. The third group included frozen and stored samples from our serum bank which had given WB an indeterminate result at the time of first determination. The meaning of an indeterminate result can be as much a sign of an infection with production of only some antibodies, partial production of various antibodies, or of non-specificity such as cross antibody reaction, etc. [1,32]. In these cases molecular diagnostic methods are employed. In our study about half of these samples belonged to patients with an acute infection and positive viraemia. The VITROS HIV Combo assay (as well as the other assays) correctly identified all these samples as well as the four HIV positive cases (even if indeterminate to WB) belonging to patients in anti-retroviral therapy. The samples from patients with acute infection, however, already had antibodies (as is evident also with the 3rd generation test for antibodies only) and this is a limitation of this study, but no samples with only antigen and without antibodies were present in our serum bank. Furthermore, it was unfortunately not possible to identify the time elapsed between sample collection and time of presumed infection because the majority of cases involved patients at risk with multiple and continuous sexual exposures. On the other hand, the effectiveness of this assay in detecting early infections, using commercial seroconversion panels, had already been highlighted in a previous work [33].