Analysis of Test Availability
The reported prevalence of the various diagnostic tests in office laboratories is shown in Table 3. The dipstick is the most common test; more than 90% of respondents reported having it in their offices. The culture was the least available test, and the prevalence of both the microscopic UA and wet prep were intermediate between the culture and the dipstick. Simple unadjusted comparisons indicate that for each test physicians’ offices in Pennsylvania were less likely to report that they have the test available than were physicians in the unregulated states (P <.0001).
Table 4 shows the determinants of test availability from the multivariate logistic regressions. After controlling for possibly confounding factors, physicians from Pennsylvania were much less likely to have each of the tests in the office. The odds ratios (ORs) range from 0.20 (for the microscopic UA) to 0.35 (for the dipstick). Each of these results is statistically significant.
A number of control variables were significantly related to test availability.* Important findings include the effects of the physician’s specialty, clinical beliefs, and the estimated number of patients with dysuria seen per week. General internal medicine physicians were less likely than family practitioners to have each of the tests in the office (OR = 0.14, 0.60, 0.35, 0.57 for the dipstick, micro UA, wet prep, and culture; all P values <.05), while obstetrician-gynecologists were significantly less likely than family practitioners to have a dipstick, micro UA, and culture (OR = 0.14, 0.24, 0.44; all P values <.05) but more likely to have a wet prep available (OR=5.46; P=.0001). Increased belief in the importance of the leukocyte eserase test for diagnosing UTIs was associated with an increase in dipstick availability (OR=3.40; P=.0001), and increased belief in the importance of the microscopic white cell and red cell readings for diagnosing UTIs was associated with an increase in micro UA availability (OR = 1.95, 1.83; P = .01, .001, respectively). Seeing an additional patient per week with dysuria was associated with an increased availability for the dipstick (OR=1.12; P=.04), micro UA (OR=1.06; P=.01), and wet prep (OR=1.06; P=.02).
The overall explanatory power of these models was fairly good. The C-statistic, representing the area under the receiver operating characteristic curve, ranged from 0.75 for the urine culture to 0.93 for the dipstick (a C-statistic of 0.5 indicates that the model is no better than random chance at predicting the outcome, while a C-statistic of 1 indicates perfect discriminating ability).
Test Availability and the Diagnostic Approach
The relationship between test availability and use demonstrates that the availability of some tests is associated with an increase in their use, while sometimes the availability of tests is associated with an increase or decrease in the use of other tests Table 5. That is, tests will sometimes substitute for or complement other tests when diagnosing UTIs. Physicians with a microscopic UA in the office were more likely to report ordering a microscopic UA. The presence of the urine culture increased microscopic UA use, but the dipstick, when available, appeared to substitute for the microscopic UA.
The reported frequency of ordering urine cultures was increased by the availability of a culture but was decreased when respondents reported having a microscopic UA or dipstick available.
The dipstick was the only test for which use was not significantly related to the presence of any of the diagnostic tests; the availability of the dipstick appears to increase its use, but this relationship was not significant.
Even though test availability is often related to use, the physician may not be the person actually doing the tests. Of the 1492 physicians who report sometimes or usually ordering microscopic UAs, 363 do the procedure themselves, and 803 use ancillary personnel in their office. The remainder (326) have the procedure done outside the office. In contrast, only 25 of the physicians who report sometimes or usually ordering a urine culture do the test themselves. Three hundred forty-two use ancillary help in the office, and 243 have the culture done at an outside laboratory.
Several physician and practice characteristics were significantly related to the frequency of ordering each of the diagnostic tests. Being an obstetrician-gynecologist relative to family practitioner decreased microscopic UA use (OR=0.41; P=.0001), while a greater belief in the value of the leukocyte esterase test decreased microscopic UA use (OR=0.70; P=.01), and greater belief in the microscopic white cell reading increased use of the microscopic UA (OR=3.12; P=.0001). Increased use of urine cultures was associated with being in a city with a population greater than 100,000 relative to a town of less than 10,000 (OR=1.54; P=.02) and being a government employee relative to being in private practice (OR=9.31; P=.0001), while culture use was decreased in practice sizes of more than 25 physicians relative to being in solo practice (OR=0.39; P=.004). The dipstick was used more commonly when physicians had a greater belief in the value of the leukocyte esterase (OR=5.15; P=.0001) and nitrite (OR=2.68; P=.0001) results. Interestingly, the physician’s specialty was generally not significantly related to test use (apart from the difference between obstetrician-gynecologists and family practitioners in micro UA use), in contrast to the consistent specialty differences found for test availability.