What you need to look out for is the number of mIU/ml the pregnancy test is measuring...

The lower the mIU/ml, the more sensitive the test...and therefore the earlier you can test!

Some tests are 10 mIU/ml...most are around 25 mIU/ml...some are more like 50 mIU/ml

Here's what wikipedia says:

All commercially available tests for pregnancy look for the presence of the beta subunit of hCG or human chorionic gonadotropin in the blood or urine. They are usually performed after a missed menstruation or 2-3 weeks after ovulation.
hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization.[1] Some home pregnancy tests claim to detect hCG as early as 4 days before the next expected period. Whether that statement is accurate for an individual woman depends on the length of her luteal phase and on how soon that individual embryo implanted. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine tests have published detection thresholds between 20 and 100 mIU/mL, depending on the brand.[2] Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests.
The really sensitive ones can be ordered through lullaby conceptions (10mIU)

Good luck!!!