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Blood pregnancy tests used by doctors

Your doctor can give you a blood pregnancy test as early as 11–14 days after ovulation. To perform a blood pregnancy test, the doctor draws blood from a vein in your arm. This blood is sent to a laboratory for testing. The results of most blood pregnancy tests take at least a couple of days. The laboratory then advises the doctor of the result.

Blood test results are about 99 per cent accurate and can detect lower amounts of hCG than urine pregnancy tests. The two main types of blood pregnancy test include:
Quantitative blood test – measures the exact amount of HCG in the blood and can give you an estimate of how far along the pregnancy has progressed.
Qualitative blood test – only checks for the presence of HCG. Since this test doesn’t measure the exact levels of HCG, it can’t offer an estimate of gestation.

http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Pregnancy_testing 

Pregnancy Test Accuracy
The accuracy of pregnancy tests has improved over the years. Both urine and blood pregnancy tests are surprisingly accurate ' in fact, they claim to produce results that are 97% to 99% accurate. Of course, mistakes can happen and it accuracy often depends on how the test was performed.

http://www.epigee.org/pregnancy/pregnancy_tests.html