PHILADELPHIA — The quality of women's sex lives has nothing to do with the amount of testosterone in their blood, a new study concluded.

This might seem to be bad news for the scores of drug companies developing testosterone products in hopes of tapping the estimated $1 billion market for a female counterpart to Viagra.

"Our study is going to frustrate a lot of doctors," said the lead author, Susan Davis, an endocrinologist at Monash Medical School in Australia. Treating female sexual complaints "is not just about giving testosterone. It's far, far more complicated."

But the Australian researchers who conducted the study are not counting testosterone out. Although levels in the blood may not be meaningful, they concluded, levels in the brain, bone and fat — where the hormone can't be measured — may affect the female libido.

"Our results are not in conflict with testosterone being used" to treat women bothered by low libido, they conclude, but neither do the results support its use.

The study appears in today's issue of the Journal of the American Medical Association.

In December, advisers to the federal Food and Drug Administration voted unanimously that Procter & Gamble had not proved the safety of its experimental testosterone patch, Intrinsa. A majority also judged Intrinsa's effectiveness to be unproved in studies, since a placebo also perked up menopausal women's sex lives.

Procter & Gamble is still seeking to market the patch in the United States, Europe and Canada, company officials said Monday. As for the Australian findings, P&G physician Dr. Kathryn Wekselman said hormone levels do not always correlate with symptoms.

"If you look at estrogen levels, you cannot predict which menopausal women will have hot flashes," she said. "So if that [disconnect] was the same with testosterone, it would not be unprecedented."

But although estrogen clearly works for hot flashes, testosterone is unpredictable for women bothered by lagging libidos. In studies of Intrinsa in postmenopausal women, for example, patch users had on average one more "satisfying" sexual episode per month, over six months, than women on a placebo; in a second study, the patch made even less difference.

Still, doctors have for decades prescribed testosterone creams and pills to improve female sex drive, particularly in women who have undergone sudden menopause because of a hysterectomy. The FDA permits such "off-label" treatment.

The Australian study involved 1,021 women ages 18 to 75 who completed a questionnaire on sexual well-being developed by Procter & Gamble. (Although P&G did not fund the study, Davis has worked on the Intrinsa research.)

Although Davis and her team found no relationship between low testosterone levels and low libido, they did find that women with low sexual function often had low levels of the hormone dehydroepiandrosterone sulfate, or DHEAS, a precursor compound that is converted by cells into testosterone and estrogen.

However, there were too many women with normal sexual function who also had low DHEAS levels to use the measurement as a diagnostic tool.

Material on DHEAS was provided by the Los Angeles Times.