What can cause a negative result on a drug screen when someone has taken the drug?

The reasons are too numerous to outline. False negatives occur in 10-40 % of drug screens largely dependent on the manufacturer, testing facility and specific drug tested for. The underlying cause is the two-tiered approach to drug testing utilized by laboratories paired with requesting party failing to specify test parameters. What type of drug do you believe was missed?

The drug was Hydrocodone. The test was done at a professional lab.

Detection of Hydrocodone requires a specific test. It would not be picked up on the standard opiate panel. It is easy to miss even when specifically tested for unless the test is ordered for "Level of Detection" (LOD), which means the instrument is set to the lowest possible setting.

The above requires a test by GC/MS and not simply a drug screen by immunoassay (IA).

Laboratories use standard screening protocol unless advised otherwise. Where it was performed is of far less consequence that what was performed.

If you have a printout or test order we can determine the test performed.

As far as I know it was just a standard test. I don't have a printout or copy of order.