Hawai'i's two largest drug-testing companies saw decidedly different rates of marijuana use among Hawai'i workers and potential employees for the first three months of the year.

Diagnostic Laboratory Services Inc., which calls itself Hawai'i's largest drug-testing company, has seen positive marijuana tests rise for five straight quarters — to 2.1 percent of more than 8,000 tests from January through March.

"Marijuana's been steadily increasing," said Carl Linden, Diagnostic Laboratory's scientific director, toxicology.

But Clinical Laboratories of Hawai'i, which conducted about 8,000 tests in the same period, has seen marijuana use steadily fall in the past two years — to 1.95 percent for the first quarter of this year.

"There's a clear downward trend in marijuana use," said Clifford Wong, forensic toxicologist and lab director at Clinical Laboratories of Hawai'i.

The different results might come from the different populations being tested.

Diagnostic Laboratory Services administers tests for about 700 companies, primarily on O'ahu. Clinical Laboratories calls itself the state's largest Neighbor Island drug-testing company and tests county employees on Maui and the Big Island and state and city workers on O'ahu, among other workers.

Overall, Diagnostic Laboratories saw positive results in 3.6 percent of its first-quarter tests; Clinical Laboratories had positive results in 4.24 percent of its first-quarter tests.

Some of the tests are given to existing workers at random. But the majority conducted by both companies are given as a requirement before being hired.

"In most situations, the individuals know they're going to be tested," Linden said. "It's surprising you'd get any positives at all."

The positive drug tests — particularly for marijuana, which can stay in a person's system for weeks — could be another result of Hawai'i's worker-friendly job market, which continues to have one of the lowest unemployment rates in the country, Linden said.

"It may allude to the short supply of labor: 'So what if I test positive? I'll just apply at another company where they don't drug-test,'" Linden said.

Diagnostic Laboratory also found that the use of crystal methamphetamine, or "ice," dropped from 1.5 percent in the first quarter of 2006 to .9 percent for the first three months of this year. The .9 percent rate was identical to results for the last quarter of 2006.

But ice use increased in Clinical Laboratories' tests — from .65 percent in the first quarter of 2006 to .99 percent this year.

Cocaine use dropped slightly in Diagnostic Laboratory's tests — from .5 percent in the first quarter of 2006 to .3 percent this year. The first quarter results were a slight increase from the .2 percent recorded in the last three months of 2006.

Clinical Laboratories saw cocaine use increase from .35 percent in the first quarter of 2006 to .4 percent this year.

But the use of opiates fell from 1.02 percent in the first quarter of 2006 in Clinical Laboratories' tests to .9 percent in the first three months of this year.

Diagnostic Laboratories has also seen positive tests of opiates fall — to .3 percent for the first quarter of this year.

Reach Dan Nakaso at dnakaso@honoluluadvertiser.com.