In a heterogeneous enzyme immunoassay, if the patient sample produces more color than the highest positive control, what action should be taken?

• A. Dilute the patient sample.
• B. Report the results out as determined.
• C. Repeat the assay using one-half the volume of the patient sample.
• D. Report the results as falsely positive.

Answer: (A)

In a heterogeneous enzyme immunoassay, the color signal is only reliably quantitative within the assay’s linear dynamic range. If the patient sample produces more color than the highest positive control, the concentration lies above the upper limit of that range, so the result can’t be quantified accurately as is. The correct action is to dilute the sample and rerun the assay, then multiply the measured value by the dilution factor to get the true concentration. This brings the measurement into the calibrated range and ensures accuracy. Reporting the undiluted result would overstate the value, and simply changing the volume without applying a proper dilution won’t fix the measurement.