The American College of Obstetricians and Gynecologists (ACOG) has issued advice on recognizing and avoiding false-positive human chorionic gonadotropin (hCG) test results. In its latest Committee Opinion report, the college acknowledges that "clinically significant false positive hCG test results are rare," citing one study in which five out of 162 women studied had evidence of serum assay interference sufficient to provide misleading results. But the college points out that such results, if unrecognized, can lead to significant and unnecessary clinical intervention.
Reviewing the causes of false-positive hCG results, the ACOG reports that some individuals have circulating factors in their serum that interact with the hCG antibody. The most common of these are heterophilic antibodies—human antibodies that can target animal-derived antigens used in immunoassays. People who have worked as animal laboratory technicians or in veterinary facilities, or who have grown up on farms, are more likely than others to have developed heterophilic antibodies, the college says.
What is a misleading serum test result? The ACOG states: "If results are misleading, they usually are seen with values below 1,000 mIU/mL." To rule out the presence of heterophilic antibodies or other interfering substances, it lists four methods:
1.A urine test: heterophilic antibodies are not present in urine.
2.Rerunning the assay with serial dilutions of serum: lack of linearity confirms assay interference.
3.Preabsorbing serum: some laboratories can pre-absorb serum to remove heterophilic antibodies before the assay is performed.
4.Using another assay: some assays may recognize the aberrant, non-active forms of hCG that some individuals naturally produce. "Repeating the hCG measurement in a different assay system can best detect this problem," says the college.
Concluding, it states: "Caution should be exercised whenever clinical findings and laboratory results are discordant. Although false-positive hCG results are rare, if unrecognized they may lead to unwarranted clinical interventions for conditions such as persistent trophoblastic disease. The physician must judge whether the risks of waiting for confirmation of results outweighs the risks of failing to take immediate action."