Errors due to carryover of analyte from a sample with a high level of analyte to a subsequent one with a low level is a potential problem with automated analyzers. Carryover is especially important with analytes such as hCG, TSH, and antibodies to HIV and the hepatitis viruses.

Carryover may be studied by using two samples, one with a high concentration of hCG (such as a woman in the second or third trimester of pregnancy) and one without detectable hCG. The low level hCG sample is placed in the first and third positions and the high hCG level sample is placed in the second position. Carryover will cause the value of the third sample to be higher than the first. Percent carryover can be calculated by subtracting the value of the first sample from the value of the third sample, dividing by the first sample value, and multiplying by 100. (3rd -1st)/(1st x 100).

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