A requisition is received for urine assays and the sample has been acidified with HCl to pH 1.5-5.0 before testing. Which analyte would now have to be rejected?

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Multiple Choice

A requisition is received for urine assays and the sample has been acidified with HCl to pH 1.5-5.0 before testing. Which analyte would now have to be rejected?

Introducing hydrochloric acid into the urine adds chloride ions to the sample. The chloride assay specifically measures Cl−, so the extra chloride from the acid would make the result falsely high and not reflective of the patient’s urine. That interference makes the chloride report invalid, so this analyte must be rejected. The other analytes—calcium, magnesium, and phosphate—aren’t directly contaminated by the added chloride in a way that universally invalidates their measurements, so they can still be tested if the assay method tolerates the acidified matrix.

Introducing hydrochloric acid into the urine adds chloride ions to the sample. The chloride assay specifically measures Cl−, so the extra chloride from the acid would make the result falsely high and not reflective of the patient’s urine. That interference makes the chloride report invalid, so this analyte must be rejected. The other analytes—calcium, magnesium, and phosphate—aren’t directly contaminated by the added chloride in a way that universally invalidates their measurements, so they can still be tested if the assay method tolerates the acidified matrix.

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