A false positive antiglobulin test can be caused by which of the following?

• A. Bacterial contamination of reagents
• B. Insufficient washing
• C. Incorrect incubation
• D. Low ionic strength solution

Answer: (A)

In this test, a false positive arises when red cells clump without the specific coating antibodies or complement you’re trying to detect. Bacterial contamination of reagents is a classic cause because bacteria present in the reagents can cause nonspecific cross-linking of red cells or interact with the anti-human globulin reagent, producing agglutination that looks like a true positive even though there isn’t a relevant antibody binding to the cells. Keeping reagents sterile and free of microbial growth is essential to prevent this artifact. The other factors mainly affect how readily the reaction occurs under proper conditions—washing, incubation, and ionic strength can influence sensitivity and actual results, but they are less likely to produce a false positive driven by contaminant microbes.