How Physicians Diagnose Urinary Tract Infections: The Potential Influence of Laboratory Regulations on Test Availability and Use

The overall power of each of these models to explain the frequency of test use was moderately good. The C-statistics ranged from 0.69 for ordering a urine culture to 0.82 for ordering the dipstick.

Discussion

This study has 2 main findings. First, for each of the office laboratory tests examined, we found significant differences in test availability between unregulated states and Pennsylvania, a state with a long-standing physician office laboratory regulatory system. The overall pattern is one of diminished test availability in Pennsylvania, after controlling for other explanatory influences. This finding was present for every test we examined, regardless of the cost of performing the test or the level at which it was regulated. This supports the hypothesis that tests are available less often in a regulated state.

The second main finding is that the physician’s diagnostic approach to UTIs was related to the in-office availability of tests. This finding supports our second hypothesis. However, the relationship between test and availability and use was more complex than suspected. Not only was the self-reported use of some tests (microscopic UA and culture) related to their availability, sometimes the availability of a test was associated with a change in the self-reported frequency of using other tests. Thus, some tests appear to substitute for others (availability of the dipstick reduces use of microscopic UAs and cultures, and availability of the microscopic UA decreases culture ordering), while the availability of the culture was associated with increased use of the microscopic UA. We also found significant relationships between the physician’s belief in the usefulness of test results in diagnosing UTI with both the availability and subsequent use of the diagnostic tests.

The multivariate analyses indicated that physician and practice characteristics were related to the availability and use of diagnostic tests. This finding is consistent with previous studies reporting numerous factors influencing the use of laboratory tests.^21-23 To our knowledge, the association between physician belief in the value of the test in diagnosing a condition and test availability and use has not been documented previously. Thus, the influence of quality regulations on test prevalence and use will be modulated by the configuration of these other factors.

This study differs from previous studies on laboratory use in that we have been able to control for many relevant explanatory variables, including the physician’s belief in the diagnostic importance of elements from the patient’s history, physical examination, and test results. Many of these items add significant predictive power in a “logical” direction. For example, the availability of many tests was related to the physician’s belief in the value of the results provided by that test, and these beliefs were related to the physician’s diagnostic approach. Although it is not possible to tell from the survey if the physician’s belief in the usefulness of the test is causally related to the availability of a test, the results indicate the need to control for physician-specific opinions and beliefs when examining laboratory test use.

It is not possible to determine the net effect of quality regulations on patient welfare from this study. In theory, a reduction in test availability and subsequent use of a less accurate diagnostic approach is offset by higher quality and accuracy in those offices still offering tests.²⁴ We did not measure the quality or accuracy of the physician’s laboratory testing. To the extent that poor-quality laboratories are closed, regulations should improve overall welfare. Alternatively, if the reduction in test availability occurs in both good- and bad-quality laboratories, then the net effects on patient outcomes may be negative. A recent cost-effectiveness study of tests used to diagnose UTIs²⁵ indicated that the microscopic UA was reasonably cost-effective ($2964 per quality-adjusted life month), but the low sensitivity of the dipstick resulted in a poor cost-effectiveness ratio ($48,460 per quality-adjusted life month). Thus, to the extent that the highly prevalent dipstick is substituting for more cost-effective tests, as our results suggest, regulations might lead to less cost-effective care.

Our analysis suggests that changing the availability of tests may change physicians’ diagnostic approaches in ways that are difficult to predict a priori. For example, reducing the availability of the microscopic UA might lead to greater use of the more expensive urine culture. When many tests are reduced in availability, it is difficult to anticipate how the care of patients will change, since tests will substitute for or complement the use of other tests in ways that are difficult to predict.

Limitations

This study has several limitations. First, it is based on survey data that may not reflect actual behavior. However, the results are plausible, internally consistent, and consistent with previous research findings documenting substantial variation in practice patterns in managing UTIs.²⁶ Our focus on how physicians treat an uncomplicated basic UTI limits the possibility of eliciting responses based on unmeasured patient characteristics. Second, the overall response rate was low, which might raise concerns that the respondents represent an atypical or biased group of physicians. A comparison of respondents and nonrespondents along the 3 characteristics we had access to did not reveal any striking differences. And, when compared with nationwide physician work force statistics collected by the AMA in 1994, the physicians in this study had similar hours per week spent in patient care (46.2 in our survey vs 48.2 in the AMA survey) and estimated level of capitation (19.3% in our survey vs 16.7% in the AMA survey).²⁷ The estimated volume of patients with UTI was also similar to previous surveys of primary care physicians (approximately 6 per week).²⁶ Thus, the respondents in this survey do not seem to represent an atypical or unusual group of physicians. Other limitations include the fact that physicians responding to the survey might not be responsible for deciding whether to acquire or maintain tests in the office. Likewise, the survey does not contain information on other factors in the physician’s local market that might influence the supply or demand for office tests. These effects, however, would have to be quite large and disproportionately affect Pennsylvania respondents to confound our findings.