The test in the lab are much more precise. They are more sensitive and can test for a greater amount of substances. The lab has less of a chance to get a false positive or false negative.Here is what wikipedia says about lab testing:
Laboratory-based drug testing is done in two steps. The first step is the screening test, which is an immunoassay based test applied to all samples. The second step, known as the confirmation test, is usually undertaken by a laboratory using highly specific chromatographic techniques and only applied to samples that test positive during the screening test. Screening tests are usually done by immunoassay (EMIT, ELISA, and RIA are the most common). A "dipstick" drug testing method which could provide screening test capabilities to field investigators has been developed at the University of Illinois.
After a suspected positive sample is detected during screening, the sample is tested using a confirmation test. Samples that are negative on the screening test are discarded and reported as negative. The confirmation test in most laboratories (and all SAMHSA certified labs) is performed using mass spectrometry, and is precise but expensive. False positive samples from the screening test will almost always be negative on the confirmation test. Samples testing positive during both screening and confirmation tests are reported as positive to the entity that ordered the test. Most laboratories save positive samples for some period of months or years in the event of a disputed result or lawsuit. For workplace drug testing, a positive result is generally not confirmed without a review by a Medical Review Officer who will normally interview the subject of the drug test.
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