A spot check of 13 drug-testing laboratories found error rates as high as 100 percent for the detection of certain drugs in urine samples, the Journal of the American Medical Association reports.

The mistakes "may threaten the treatment process" for some drug abusers and "reflect serious shortcomings in the laboratories."

The Centers for Disease Control sent drug-spiked urine samples to the labs and disguised them as regular samples from methadone treatment centers. The laboratories serve a total of 262 such treatment centers.

Error rates -- failure to detect the drug -- ranged from 11 to 94 percent for barbiturates, 0 to 33 percent for methadone and 0 to 100 percent for cocaine. In tests for most of the drugs, only one or two of the 13 labs were judged "acceptable."

Labs may fail to detect drugs because people are hoping there are no drugs in the samples. "Negative results are an indicator of successful treatment and the compliance of the patient," the journal suggests.

Results showing that no drugs are present also "justify the public expenditures" for drug-treatment programs, the report says.

Some labs reported false positives -- the presence of a drug that wasn't there -- in as many as 66 percent of the samples.