On the issue of defensive medicine, which is often cited as a major health care cost escalator, GAO says that the overall prevalence and costs of this practice have not been reliably measured.

Studies designed to measure defensive medicine, GAO says, focus on physician behavior in specific clinical situations.

Because of this limited scope, GAO says, the study results cannot be generalized to estimate the extent and cost of defensive medicine practices across the health care system.

The new GAO report, Number 03-836, is the second major analysis released by the Congressional watchdog in the past few months. An earlier report, number 03-702, which drew strong praise from industry groups, said that increased costs, rather than insurance industry practices, are largely responsible for medical malpractice premium rate hikes.

But industry groups were less praiseworthy of the new report. Larry Akey, a representative of the Washington-based Health Insurance Association of America, said that the new GAO report reinforced what HIAA has been saying all along, which is that high medical malpractice insurance rates are affecting access to selected health care providers in some locations.

However, Mr. Akey said, the GAO sets a very high standard in its discussion of the impact of defensive medicine.

Study after study has shown that defensive medicine is a major factor in driving up insurance costs, he said. While the studies may not be as concrete as GAO would like, Mr. Akey said, all the studies point in that direction.

In the new report, GAO focuses on five states that have reported access problems due to medical malpractice costs-- Florida, Nevada, Pennsylvania, Mississippi and West Virginia. GAO says it confirmed instances in these five states where malpractice pressures led physicians to take actions that reduced access to services involving emergency surgery and newborn deliveries.

However, GAO says, these instances were not concentrated in any one geographic area and often occurred in rural locations, where maintaining an adequate number of physicians is a longstanding problem.

Moreover, GAO says, many of the reported provider actions relating to malpractice pressures, such as relocating, were either unsubstantiated or did not widely affect access to health care.

GAO acknowledges, however, that it is essential to continue monitoring provider responses to rising medical malpractice premiums.