Home pregnancy tests give women the opportunity to determine whether they are pregnant within the privacy of their own homes. These tests have become so popular that they are now the most commonly used over-the-counter disposable health kits. Home pregnancy tests were introduced in 1975 and, since then, more than 20 brands have become available. These testing kits use modern immunometric assays, and the manufacturers claim that the tests can be used on the first day of the missed period. In addition, the manufacturers claim that the tests are accurate more than 99 percent of the time. Because these tests are used to confirm an intended or unintended pregnancy, verifying the accuracy of these claims is important. Cole and colleagues assessed the validity of the claim of more than 99 percent accuracy of home pregnancy tests on the first day of the missed period.
In the first phase of the study, urine human chorionic gonadotropin (hCG) levels were assessed during early pregnancy by checking urine samples from women who were between four weeks and zero days and four weeks and three days from their last menstrual period and were pregnant. This information was used to statistically determine the mean, 5th, and 95th percentiles of hCG concentration.
The next step was to test each of the 18 brands of home pregnancy tests using five concentrations of urine hCG. Each brand was tested six different times. The urine concentration of hCG was established by adding recombinant hCG in concentrations of zero, 12.5, 25, 50 and 100 mIU per mL. Sensitivity was defined as the urine concentration at which six of six tests gave positive results at the suggested or extended time.
The calculated mean urine hCG concentration after missed menses was 49 mIU per mL, the 5th percentile was 12.4 mIU per mL, and the 95th percentile was 241 mIU per mL. The home pregnancy tests had to be able to detect a urine concentration level of 12.4 mIU per mL to confirm intrauterine pregnancies 95 percent of the time. To be 95 percent accurate at days 1, 2, and 3 after missed menses, the concentration of urine hCG had to be 21, 35, and 58 mIU per mL. Of the 18 brands included, only one test had a sensitivity that detected the 12.5 mIU per mL level. If read at the suggested time, only 44 percent of the brands could detect urine hCG concentrations at 100 mIU per mL. When extended time was allowed or a response that was “faintly” positive was considered, all 18 brands were positive at 100 mIU per mL. The authors estimated that 100 mIU per mL was equivalent to the 84th percentile at the time of the missed period, so these tests would detect only about 16 percent of pregnancies.
The authors conclude that the utility of home pregnancy tests is questionable and that clinicians should be aware of the limitations of these tests. They recommend that these issues be addressed because a missed diagnosis of pregnancy can have a significant impact on prenatal care and changes in medications and behaviors that are consistent with healthy pregnancy.