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New roles for the urine dipstick; chemical screening programs can reduce the volume of unnecessary microscopic exams and urine cultures.

New roles for the urine dipstick

Examination of urine is the oldest laboratory test known, dating back to the ancient Babylonians and Sumerians. It is still popular today--chemical, microscopic, and bacteriologic urine studies are among the most commonly requested laboratory tests and account for a substantial share of laboratory charges.

An estimated 225 million urinalyses are performed in the United States each year. An average patient undergoes two urinalyses per hospitalization.

Urine cultures, in turn, may represent as much as 50 per cent of the specimens examined in a clinical microbiology lab. The labor-intensive nature of microbiology makes it difficult to reduce the cost of culturing urine. This is particularly frustrating considering that, in many hospitals, as few as 10 per cent of specimens reveal significant bacteriuria.

The specter of prospective payment has brought the cost of urine examination under new scrutiny. Pressure to reduce spending is likely to increase in the future, as administrators realize that repeated use of such inexpensive tests as urinalysis and urine culture adds far more to hospital costs than the occasional use of high-priced technologies such as the CAT scan.

Two proposals for reducing the cost of urine testing have recently been promulgated in the literature: Use the least expensive urine test--the dipstick or chemical analysis--to screen specimens for microscopy and for culture.1-4 The new roles for the urine dipstick have provoked considerable controversy.

Proponents contend that dipstick screening is both safe and cost-effective. They argue that with the introduction of nitrite and leukocyte esterase reactions, dipsticks are now able to flag pyuria and bacteriuria that would previously have been missed with older reagent strips. By restricting urine microscopy and culture to chemically abnormal specimens, more time and care can be reserved for specimens most likely to reveal abnormalities.

Opponents of dipstick screening for microscopy and culture argue that abnormalities will be missed and that patient care may suffer. Furthermore, they point out that it is the clinician's prerogative, rather than the laboratory's, to decide which tests are necessary for optimum care.

Let's examine medical and economic issues surrounding the chemical screening of urine.

Screening for sediment microscopy. This type of chemical screening program reserves sediment microscopy for specimens that first show one or more predefined dipstick abnormalities. Advocates of the various screening programs follow somewhat different dipstick screening criteria, but all concede that even their own approaches will miss some sediment abnormalities.

There are no clever algorithms that will guarantee 100 per cent dipstick sensitivity in detecting microscopic abnormalities and still allow reasonable specificity. This should come as no great surprise since the reagent strip was originally conceived and developed to complement the microscopic examination and has only recently included tests that over-lap with sediment microscopy.

In a retrospective study of 1,000 patients who underwent urinalysis, we found that 13 per cent of the specimens with some type of abnormality slipped through the screening program undetected. We also noted that a wide spectrum of sediment abnormalities --including casts, Trichomonas, bacteria, yeast, leukocytes, and red blood cells--can pass through screening unnoticed. In another study, which utilized newer dipsticks with leukocyte esterase reactions, less than 6 per cent of patients with a urinalysis abnormality were missed.2

To assess the medical consequences of specimen screening, we reviewed the records of patients with sediment abnormalities missed on a dipstick screen.3 Of course, laboratory abnormalities do not necessarily reflect clinical illness, and overlooking a particular abnormality does not automatically compromise patient care.

This became evident soon after our study began. Urine specimens from two women showed a normal chemistry, but red blood cells were detected microscopically. While hematuria can be an ominous clinical sign, a subsequent investigation revealed that both specimens had been collected during active labor and were obviously contaminated with vaginal blood. Since both women had received generous prenatal vitamin supplementation, ascorbic acid interference presumably explained the false-negative reagent stip results.

Study findings indicated that most of the sediment abnormalities missed by a dipstick screening program were of no clinical significance.3

The pathologic states of potential consequence most likely to be overlooked were bacteriuria (&<10(5) bacteria/ml) and Trichomonas infection. Yet the medical benefit of performing microscopy in these two groups of patients was not clear. While bacteria in the sediment or pyuria may result from urinary tract infection, the predictive value for infection was so low in our study that the finding was unhelpful clinically. The great majority of specimens with bacteria or white blood cells in the sediment were not associated with symptoms of urinary tract infection or significant bacteriuria upon culture.

Similarly, asymptomatic Trichomonas infection is generally not treated, because of the mutagenicity of metronidazole. Therapy is reserved for the subgroup that becomes symptomatic, and these patients are diagnosed more effectively with a pelvic examination than by urinalysis.

For these reasons, I cautiously suggest that routine sediment microscopy can be safely reserved for specimens that have shown chemical abnormalities. Several recent studies have reached similar conclusions.1, 2

Most investigators recommend sediment microscopy for patients with symptoms referable to the genitourinary tract, regardless of reagent strip results. The possibility that very low-frequency diseases may not have been represented in study populations remains a limitation of our research work and the work of others.

Will a screening program for urine microscopy save money? Almost always. Depending on practice patterns within a hospital and the particular definition of a normal chemical screen, 30 to 60 per cent of the chemical examinations will be normal. Half of our urinalysis specimens were normal on a dipstick screen.

Several authors from medium to large hospitals have calculated that a dipstick urinalysis screening program yields annual direct-cost savings in the $10,000 range.2, 3 These estimates do not include the cost of unrevealing follow-up studies prompted by sediment abnormalities that do not reflect any underlying pathology. Patients at our institution are charged an estimated $40,000 or more each year for follow-up tests and procedures that might be obviated by a dipstick screening program for sediment microscopy. It seems reasonable to conclude that screening could provide real savings to the average hospital.

Many hospitals fail to achieve the maximum benefits of a screening program because the dipstick criteria for miscroscopic examination are selected injudiciously. Not every chemical abnormality requires microscopic analysis.

For example, glycosuria can be a clinically important laboratory finding, but it is a poor predictor of abnormal microscopy. A laboratory that routinely performs microscopy on screened specimens with glycosuria will experience only a small reduction in its workload. It is essential to distinguish between the dipstick's diagnostic utility and its new role in screening for microscopy. Figure I presents a workable set of dipstick criteria for selection of appropriate microscopy specimens.

In a recent CAP survey, 17 per cent of responding hospital labs used a dipstick screen for sediment microscopy. Most of these labs construe a urinalysis order as a request for chemical screening; they do not have to teach the clinical staff to specifically ask for a chemical urine screen. If desired, microscopy can be ordered separately for specimens with a normal chemistry.

There are very few reports of clinician dissatisfaction with urine screening programs. Even under the prevalent system of routine microscopy, several studies indicate that physicians are not interested in sediment abnormalities when urine chemistry is normal.1, 3, 5

Screening for bacteriologic culture. Most studies of this approach have restricted dipstick evaluation to the leukocyte esterase and nitrite reactions, culturing specimens if either test is positive in trace quantities or greater. With these criteria and 10(5) bacteria/ml as the threshold for a positive culture, dipstick sensitivities ranging from 85 to 100 per cent have been reported.

Unfortunately, several of the studies were undermined by poor methodology. Sample sizes were generally much too small for accurate estimation of dipstick sensitivity. When as few as one in 10 specimens demonstrate significant bacteriuria, at least 9,000 specimens must be examined in order to estimate a 90 per cent dipstick sensitivity with a 2 per cent confidence interval.

What are the medical consequences of screening culture specimens? Accepting, say, a dipstick sensitivity of 90 per cent, we must ask ourselves whether we are willing to miss as much as 10 per cent of the urinary tract infections in our hospitals. Although lower urinary tract infections generally run a benigh course and may disappear spontaneously, a small subset of untreated patients will develop sepsis or pyelonephritis.

It is clear that any study evaluating the clinical consequences of screening culture specimens with a dipstick must include vast numbers of specimens, since as few as 10 per cent will actually show significant bacteriuria. Dipstick screening will catch most of these cases, and only a small proportion of the remaining patients will ultimately develop complications if untreated.

This means it would be necessary to study as many as 20,000 specimens to find 10 patients suffering complications as a result of passing through a dipstick screen undetected. It is no wonder that published reports have failed to demonstrate any adverse effects resulting from a screening program.

Evaluating chemical screens for urine culture becomes even more complicated in the light of recent reports that as few as 100 bacteria/ml may produce symptomatic cystitis in young women. If our clinical colleagues ask us to quantitate bacteriuria in this range, dipstick screening programs will be even less effective.

Considering the heavy burden that urine cultures place on the clinical microbiology laboratory and the low detection rate for significant bacterijuria, a dipstick-based screening program would be an economically attractive venture. Many laboratories can reduce urine culture specimens by more than 50 per cent. This translates into annual direct-cost savings of approximately $20,000 for a medium to large hospital. As a safeguard, one Eastern hospital that uses chemical screening holds uncultured specimens at 4C for several days to give physicians the option to culture at a later date.

Financial savings notwithstanding, I am uncomfortable with the practice of dipstick screening for urine culture. In my opinion, it has not been studied adequately. We must not lose sight of the fact that some patients with significant bacteriuria will go undetected and that a small percentage of infected patients will become gravely ill if not treated.

Some investigators argue that more seriously ill patients will not be missed because the leukocyte esterase reaction will detect the inflammatory response that accompanies tissue invasion associated with serious infection. This may or may not be the case. The available evidence indicates that a substantial portion of patients with pyelonephritis do not demonstrate pyuria.6

To sum up, the cost of urine testing is coming under closer scrutiny as a result of new financial pressures. Two novel approaches suggested to reduce test costs are to screen urine specimens chemically before performing either sediment microscopy or bacteriologic culture. While both approaches will save money, they raise serious concerns about clinical safety and the laboratory's prerogative to interpret clinician's test orders.

I cautiously support the use of a chemical screen to select urinalysis specimens for microscopic examination, but remain unconvinced about the wisdom of using a reagent strip screen prior to culturing.

We must address the real problem: Physicians often culture indiscriminately. Hospitals aren't seeing more urinary tract infection, but we are processing more cultures. Consequently, a greater percentage of the results are normal. Many laboratories have turned to imperfect screening programs to weed out such specimens and eliminate unnecessary cultures. It might make better clinical sense to spend our time discouraging indiscriminate culturing.

The laboratory is moving away from shotgun, parallel testing of urine and other specimens toward conditional, progressive testing. In the future, we will be asked to evaluate many algorithms that will purportedly reduce laboratory workload without compromising patient care. Some of these strategies depend on the medical staff's willingness to alter practice patterns and function with fewer test results.

The laboratory, in turn, must maintain a clinical orientation and work closely with the medical staff if we hope to sort out sound cost-reducing strategies from irresponsible schemes.

1. Bartlett, R.C., and Kaczmarczky, L.A. Usefulness of microscopic examination in urinalysis. Am. J. Clin. Pathol. 82:713-716, 1984.

2. Shaw, S.T.; Poon, S.Y.; and Wong, E.T. Routine urinalysis: Is the dipstick enough? JAMA 253: 1596-1600, 1985.

3. Valenstein, P.N., and Koepke, J.A. Unnecessary microscopy in routine urinalysis. Am. J. Clin. Pathol. 82: 444-448, 1984.

4. Schumann, G.B., and Greenberg, N.F. Usefulness of macroscopic urinalysis as a screening procedure: A preliminary report. Am. J. Clin. Pathol. 71: 452-456, 1979.

5. Heimann, G.A.; Frohlich, J.; and Bernstein, M. Physicians' response to abnormal results of routine urinalysis. Can. Med. Assoc. J. 115: 1094-1095, 1976.

6. Kass, E.H. Bacteriuria and the diagnosis of infections of the urinary tract. Arch. Intern.Med. 100: 709-714, 1957.

Table: Figurel Criteria for performing sediment microscopy
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Author:Valenstein, Paul
Publication:Medical Laboratory Observer
Date:Feb 1, 1986
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