Many labs, including Labcorp, will develop specific testing panels for their customers, which will allow for customization of testing according to the needs of the program.
In general, though, Labcorp follows a two step testing procedure for hair drug testing, first performing testing by using a screening test for the drugs being assessed and then performing confirmation with gas chromatography/mass spectrometry (GC/MS) on any specimens that are positive on the screening test. Some labs will routinely only perform the more accurate confirmatory test, and the client can request Labcorp to do so, but the usual mechanism for Labcorp is to perform the two step testing method.
This is important to your questions because the screening tests, whether of urine or hair, have a fair amount of inaccuracy, both with false positives and false negatives. The false positives generate the greatest concern, since it is undesirable to accuse an innocent person of drug use, which is why a confirmatory test is done on any positive result. However, the false negatives mean that someone that is taking a drug is not detected by the test, so may pass the test despite usage of the drug.
Part of the reason for the false negatives is that many of the commonly used drugs are chemically different than the primary drug in that category. For example, the test for opiates is looking for morphine, and is generally very good at detecting morphine and those drugs that are converted to morphine in the body, such as codeine and heroin
. However, the screening test is not good at detecting the synthetic or semi-synthetic opiates, such as hydrocodone
or oxycodone, so is frequently missed.
Hair drug testing also can easily miss occasional use of a drug, particularly single uses of a drug. As the use of the drug increases, then the likelihood of detection increases.
In your case, it is very possible that the test could miss the drugs that you are taking because of the false negative rate associated with the screening test.
From the standpoint of the effect of coloring the hair on testing, there is no evidence that coloring the hair will result in a negative result. There are people that claim anecdotal support of this effectiveness by having passed a test, but some people pass tests without coloring their hair, such as for the reasons noted above. There also are many people that have failed the drug test despite coloring their hair. Coloring the hair is not a reliable method for passing a hair drug test.
If I can provide any further information, please let me know.