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

• A. Inadequate washing of red blood cells
• B. Bacterial contamination of reagents
• C. Addition of AHG reagent
• D. Low temperature incubation

Answer: (A)

In an indirect antiglobulin test, after incubation of patient serum with screening red cells, thorough washing removes unbound antibodies so that only antibodies bound to the red cells remain for detection by the antiglobulin reagent. If the washing is inadequate, residual free IgG and other proteins stay in the reaction. This free IgG can bind to the antiglobulin reagent itself, reducing the amount of reagent available to crosslink red cells that are already coated with IgG. As a result, agglutination may not form even when clinically significant antibodies are present, producing a false-negative result.

The other options don’t fit this mechanism: bacterial contamination would cause broader reagent instability and unreliable results; the antiglobulin reagent is required for the test to work; and while low-temperature incubation can diminish IgG interactions and lead to false negatives, the specific cause here is the insufficient washing step.