Warning on False Positives in hCG Assays

hCG Reference Service at the University of New Mexico (USA; www.unm.edu), led by Laurence A. Cole, Ph.D., professor of obsterics and gynecology. The research was reported in The Lancet (2000;355:712-715).

The hCG measurements are wholly positive in pregnancies, trophoblastic disease, or cancer, and wholly negative in other cases. This can lead to disastrous consequences. For example, a false positive patient subsequently found not to be pregnant might therefore be assumed by her doctor to have cancer. This could result in a hysterectomy or other procedure that would leave her infertile. Thus, the researchers urge that extreme caution is needed in order to avoid false positives.

The Reference Service was consulted on 62 cases in which false positive results were suspected. Of these, 32 were being monitored by the Abbott AxSym test and the other 30 with tests of other manufacturers. Of the 24 cases identified as false positives, 23 were from doctors using the Abbott AxSym test to monitor patients and one was from a doctor using the Bayer Immuno-1 test. The results clearly show that the Abbott test produces far more false positives than other assays, say the researchers, although other assays can also give false positive values but they are lower and may lead to less aggressive therapy.

The 23 patients monitored with the Abbott AxSym test all had hCG results greater than 10 U/l (100%), while 9 of the 23 had results greater than 100 IU/l (41%). The results of other tests performed by doctors or the Reference Service on the same 23 patients showed that 11 of 57 test results were greater than 10 U/l (< 20%) and only 1 of 57 was greater than 100 IU/l (< 2%).




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