Proponents of drug screening argue that it can reduce drug-related accidents and detect drug users in the workplace. Opponents counter that drug screening invades employee privacy, may be abused and inaccurate, and creates problems between management and employees. When a positive reading results from an initial test, two or more additional levels of testing are required to pinpoint the specific drug found in the sample. This series of tests may cost up to $80 for each person being tested. Screening methods include gas chromatography, thin-layer chromatography, and mass spectrometry. One study of 13 laboratories offering these tests found error rates as high as 100 percent. Despite these problems, many employers, particularly in the transportation sector and heavy industries, believe they must test employees to protect themselves against liability damages. The question of whom to test should be resolved by a consideration of the particular occupation and the safety and security hazards involved. An additional approach used by employers is preemployment screening. Another policy issue is the question of how to handle those employees found to be using drugs: fire or rehabilitate? Companies offering drug rehabilitation programs have found this to be a cost-effective approach. However, the full impact of drug screening programs in the workplace has yet to be seen.