Faulty washers and soiled micropipettors may generate false positive serological results

Background: Serological markers such as HBsAg and anti-HIV may be present in serum at very high concentrations and this may give rise to erroneous diagnoses due to cross-contamination.

Objectives: To investigate poor equipment maintenance and use, including contamination with human serum, as a potential source of erroneous assay results.

Study design: The potential of microtitre plate washers and micropipettors to transfer material between microplate wells and between specimens was examined. For the study of micropipettors we recruited 19 UK diagnostic laboratories.

Results: Four out of seven plate washers in use, until adjusted, had the potential to cause false positive HBsAg reactions. The centering of the probes that delivered the wash fluid, delivery pressure, wash volume and the use of a pre-programmed card to direct the washing procedure were important variables. We investigated soiling of tip cones of micropipettors. In every laboratory human IgG could be detected in at least a third of eluates from micropipettor tip cones; only 31 (14%) of 222 showed no evidence of contamination with human serum. Only one laboratory submitted eluates devoid of specific antibodies. Anti-HAV was the marker most commonly found (n = 68), followed by HBsAg (n = 27) and anti-HIV (n = 20). Seven micropipettor eluates from two laboratories were radioactively contaminated.

Conclusions: Recommended precautions are regular checking, cleaning and servicing of equipment, care in interpreting weak reactions, reference back to serum left on the clot of the original specimen and testing of a follow up specimen. Poorly maintained immunoassay equipment can readily generate false positive results due to low-level cross-contamination, particularly with the current highly sensitive HBsAg and anti-HIV assays.