Interference in the autoDELFIA(R) hAFP immunoassay and effect on second-trimester Down's syndrome screening.

Background: Falsely decreased serum alphafetoprotein (AFP) concentrations are reported in the autoDELFIA(®) hAFP immunoassay due to interference by complement. AFP is measured, using this assay, as part of second-trimester and integrated Down's syndrome screening tests. Decreased AFP concentrations increase the calculated risk of Down's syndrome; therefore falsely low AFP, due to assay interference, may artificially increase a patient's risk, and have the potential to cause false screen positive results. It was our aim to assess whether negative interference in the autoDELFIA(®) hAFP assay was a cause of very low AFP concentrations, and to examine the effect of falsely decreased concentrations on the calculated risk of Down's syndrome.

Methods: Three hundred and twenty-three sequential Down's screening serum samples with very low serum AFP concentration (<15 KU/L) using the autoDELFIA(®) hAFP immunoassay were selected and AFP re-measured using the E170 AFP immunoassay.

Results: Interference was detected in nine samples (from eight patients) on the basis of discordant AFP concentrations. The interference decreased following storage of samples at 4°C to deplete complement. Use of the falsely low AFP concentrations to calculate risk of Down's syndrome resulted in significantly increased calculated risk compared with complement depleted results.

Conclusions: Laboratories should be aware that falsely low AFP concentrations due to complement interference may be obtained using the autoDELFIA(®) hAFP immunoassay. We have shown that falsely low AFP concentrations increase the calculated risk of Down's syndrome. This is a potential cause of false Down's syndrome screen positive results.