www.ncbi.nlm.nih.gov/m/pubmed/16295647/
In brief:
RESULTS: First Response Early Result had an analytical sensitivity of 6.3 mIU/mL, which was estimated to detect greater than 95% of pregnancies on the day of missed period. The sensitivity of Clearblue Easy Earliest Results was 25 mIU/mL, which indicated detection of 80% of pregnancies. The sensitivity of the five other products was 100 mIU/mL or greater, indicating detection of 16% or less of pregnancies. Three of these last products included faulty devices.
CONCLUSION: Universal claims for OTC-PTs of more than 99% laboratory accuracy and use as early as the first day of missed period, while cleared for use by the U.S. Food and Drug Administration, are ambiguous and inappropriate for many products, according to these data. The majority of products tested were found to detect only a small percentage of pregnancies on the first day of a missed menstrual period. Until more data become available on the actual clinical sensitivity of these products, pharmacists should advise consumers to be cautious in interpreting test results.