Here is a cut and past discussion from a scientif article which tested the accuracy of HPT 's. Maybe it is some answer all those false negative and inconsistent tests we all seem to have! The tests are not 99% accurate as manufacturers claim.
Home pregnancy testing was introduced almost 3 decades ago. To appeal to consumers in this compet_tive market, manufacturers have shortened the time of early diagnosis to the same day of the missed period. One brand (First Response, Early Result) has been reported to be accurate 4 days before the missed menses. Several new brands or new adaptations of existing brands, not evaluated here, now make this earlier use claim. Our research found that in many circ_mstances HPTs were not sensitive enough to detect the low hCG levels reported at the onset of a missed period, or they provided faintly discernible results at multiple test concentrations. A faintly discernible result is open to misinterpretation.
We found in well-dated pregnancies that a wide range of hCG (from 23 to 2438 mIU/mL) is produced by pregnant women by 28 to 30 days after the onset of the last known menstrual period. It was calculated that for a test to detect 95% of pregnancies at the time of missed menses, it would need to consistently detect at least 12.4 mIU/mL hCG. If a HPT brand was used at days 1, 2, or 3 after the onset of the missed menses, then to detect 95% of pregnancies it would need to consistently detect 21, 35, and 58 mIU/mL hCG, respectively. It should be noted that these values are estimates based upon regression lines from only 25 women achieving pregnancy.
All 18 HPT brands evaluated use proven multiantibody technology with very similar testing instructions. Tests were credited as being positive regardless of whether the line was clearly positive or only faintly discernible in the result window. A total of 6%, 17%, 44%, and 83% of brands tested at the suggested reading time gave clear or faintly discernible positive results at hCG concentrations of 12.5, 25, 50, and 100 mIU/mL, respectively. In contrast, only 0%, 0%, 22%, and 44% of these brands tested gave clear or indisputably positive results at the suggested reading time at these same concentrations. These results, together with the data indicating hCG concentration around the time of missing menses, clearly indicate that a high percentage of pregnant women will have a negative test result during the first few days after the missed period. We found that most tests were consistently accurate only after the urine hCG concentration was at least 100 mIU/mL, or the highest hCG concentration evaluated, allowing for both clearly positive and faintly discernible readings and allowing for readings extended to 10 minutes, rather that at the 1- to 5-minute suggested reading time. The poor findings reported here may in part be due to consideration of multiple lots of brands purchased from multiple stores and the shipping and storage variations. We estimate that 100 mIU/mL is equivalent to the 84th centile at the time of the missed menses, so that a test with this sensitivity would detect only about 16% of pregnancies.
Realizing our findings, how can manufacturers claim such a high accuracy in very early gestation? The reason for this product claim relates to an arcane Food and Drug Administration (FDA) regulation. The manufacturer needs only to demonstrate that its test results, using a nonpregnant woman's urine supplemented with commercial hCG, agree with those of an existing test more than 99% of the time. Today's HPT brands are more sensitive than previous products, so they would be expected to give as many positive results as older products. This FDA evaluation, however, has no bearing on the ability to detect early pregnancy.
In addition to the inability of these HPT brands to detect the low hCG concentrations a__sociated with very early pregnancies, other reasons may explain a negative result using these brands. The person may have erroneously calculated the time of onset of her missed period, she may have a delayed menses due to an early pregnancy loss[5.] or delayed ovulation, or implantation may have prolonged the onset of significant hCG production. [6., 7. and 8.] Although the test instruction sheets were easy to understand, an erroneously performed test or faintly discernible result may also explain a falsely negative result.
The purpose of this investigation was not to recommend specific brands of HPT. Nonetheless, we found that First Response, Early Result was the most sensitive test, consistently detecting 12.5 mIU/mL of hCG (in a supplementary study by Cole et al[9.] this HPT detected 6.3 mIU/mL hCG), considering the manufacturer's suggested read time, and both clear and faintly discernible positive results. This is the sensitivity needed to detect 95% of pregnancies at the times of the missed menses (12.4 mIU/mL). It is inferred that this test is the most likely of the 18 brands evaluated to detect a pregnancy at the time of the missed menses.
Two of the 18 brands had clear technical or design problems (Clear Choice and Confirm). These tests both gave falsely positive hCG test results with urine containing no hCG and also gave numerous invalid results as indicated by the absence of a confirmation or validity line. Such brands may be unacceptable and may need to be withdrawn and fixed. They should probably be avoided because they may generate false hope and great confusion among users.
Costs of the different HPT brands varied from $8 to $16. The more expensive brands contained a second test to be performed later, “in case the first test was negative.” This option to purchase a kit with two tests would be reasonable if the patient understands in advance that testing at the day of the missed period or earlier can yield negative or only faintly positive results (even though the user maybe pregnant). Use of two tests is also helpful in determining an early pregnancy loss (ie, test on day of missed menses is positive, whereas test a few days later is negative).
Home pregnancy testing continues to be a dominant method for early pregnancy detection despite questions about clarity of positive results, false pregnancy detection as a result of early pregnancy loss, and problems with interpretation of a negative result. Considering the a___lytic detection limits of HPTs and the wide variability in hCG concentrations, we question the greater than 99% accuracy claims made by the manufacturers. In fairness to the consumer, we suggest that these guidelines be re-examined by the manufacturers. FDA 510(k) regulations should insist on measurements of clinical sensitivity for detecting pregnancy on the day of the missed menstrual period and at other specific time points, as support for an accuracy claim. By doing so, a clearer understanding of diagnostic accuracy can be reported on each brand's package insert rather than the printing boldly “over 99% accuracy” on the outside of the package. All these issues need to be considered to avoid delays in detection of pregnancy, not only for early commencement of prenatal care, but to allow for appropriate changes in medication and behavior consistent with healthy pregnancy (avoidance of alcohol, tobacco, and inappropriate medicines), or to seek earliest pregnancy termination if desired.