Evaluation of a Scoring System for Leukocyte Esterase-Nitrite Dipstick Screening for Urine Culture

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1 UILSIS Evaluation of a Scoring System for Leukocyte Esterase-itrite Dipstick Screening for Culture Gifford Lum, MD, and William. Thiemke, PhD published scoring system based on four performance characteristics ofa laboratory test sensitivity, specificity, and predictive value ofa negative and a positive test was evaluated to determine whether it could be applied to the problem ofeliminating routine urine culture based on leukocyte esterase-nitrite () dipstick urine findings. Equal weight was assigned to each ofthe four performance characteristics, and a scoring scale ofl 00 was used for simplicity. positive urine culture was defined as more than s colony-forming units per milliliter. studies from the medical literature were assigned, based on the published scoring system, into one ofthree categories: accept, more than 0; conditional accept, 0 to 0; and reject, less than 0. greement between the published scoring system and author (empirical) acceptability was good for urine studies with scores of greater than 0 or less than 0. greement for urine studies with scores between 0 and 0 was not good ( 0% disagreement), which lead us to designate this range as "equivocal." The published From the Laboratory Service, Brockton/West oxbury Veterans dministration Medical Center and Harvard Medical School, Boston, M scoring system used for assessing the feasibility ofeliminating routine urine microscopy based on dipstick urine findings cannot be applied without modification to urine studies that assess the feasibility ofeliminating routine urine culture based on dipstick findings. The following set ofrecommendations are proposed as a guide to provide a rational basis for accepting or rejecting screening for urine culture: accept ifthe score is more than 0; equivocal ifthe score is 0 to 0; accept ifcost savings (reagent and technician time) is ofprimary importance, reject ifsensitivity is unacceptably low; and reject ifthe score is less than 0. rine cultures represent a significant work load for most microbiology laboratories in terms of the total number of specimens received and the proportion of weighted work load. Because the majority of requested urine cultures are usually negative and because conventional techniques for quantitative urine culture have a turnaround time of to hours, much effort has been ex- U October Downloaded from on February 0 pended tofindmethods to streamline the screening process in an attempt to separate urine specimens into two groups those that require and those that do not require urine culture. One recent technique, suggested to provide a rapid screening for bacteriuria, is based on the physiochemical dipstick test for the presence of urinary nitrite () and/or leukocyte esterase () activity. lthough much has been written regarding the feasibility of screening urine cultures beforehand using this method, there have been no attempts to assess objectively whether the laboratory can eliminate culturing a urine specimen based on a negative biochemical dipstick result. We decided to assess the feasibility of using dipstick screening for urine culture using a recently proposed scoring system for evaluation of dipstick urine studies. Using the published scoring system, originally intended to assess the feasibility of eliminating the urine microscopic examination based on urine dipstickfindings,we assigned a cumulative weighted score to urine literature studies and categorized each study according to author (empirical) acceptability. We then compared this cumulative score to the published model to determine whether there was any agreement

2 and whether one could apply a scoring system designed to evaluate elimination of urine microscopic examination to the problem of screening urine specimens for culture. scores. Thus, a "perfect" urine study would have a score of 0. studies could then be ranked according to each performance characteristic as well as by overall cumulative weighted score. Selection of Studies From the Medical Literature literature search was conducted using the following key words in a Medline computer search: urinalysis, urine culture, nitrite, leukocyte esterase, and bacteriuria. Screening was usually done by title, andfinalselection for inclusion into this study was made on the basis of whether was an essential part of the data analysis and whether the following data could be abstracted from each study sensitivity, specificity, and P Vneg and PV tests. total number of urine studies were selected and assigned identification numbers corresponding to the reference bibliography citation. M aterials and Methods Scoring System for Evaluation of Screening We adopted the scoring system previously published by Lum and Morrison. Equal weight () was assigned to the four performance characteristics of a good laboratory test, namely, sensitivity, specificity, predictive value of a negative (PVneg) test, and predictive value of a positive (P V ) test. In place of the false-positive and false-negative rates used by previous authors, we used the equivalent definition of PV for ( false-positive rate) and PVneg for ( false-negative rate). To obtain the weighted value for any performance characteristic, one would multiply the weighted factor of by the percentage of sensitivity, specificity.pv^orpv^. fter assigning a weighted value to each performance characteristic, a summation procedure was then used, obtained by adding individual weighted ssessment of Performance Characteristic of the Laboratory Test Definitions and abbreviations adopted for this study in order to provide consistency and standardization are found below. "disease" category is a positive urine culture usually based onfindingmore than colony-forming units per milliliter, or in some cases as otherwise defined in each urine study. "positive" urine dipstick is the criterion adopted in each urine study for a positive and/or reaction. true-positive (TP) result is a positive urine culture and a positive and/or true-negative (T) result is a negative urine culture and a negative and/or false-positive (FP) result is a negative urine culture and a positive and/or false-negative (F) result is a positive urine culture and a negative and/or Sensitivity was calculated by the following: TP/(TP + F); specificity, T/ (T + FP); P Vncg test, T/(T + F); and PVpos test, TP/(TP + FP). Published Scoring System Model for Evaluation of Studies Using the cumulative weighted score (scale of 0), we proposed the following model for assigning urine studies to ac- Table I: Summary and anking of Individual Studies ( and/or ) Within a Defined Test Performance Characteristic Category (%, Weighted Value)* ank Study o. B Sensitivity (0%, (.%, (.%, (.%, (0.0%, (6.6%, (6.%, (6.%, (.0%, (.%, (.%, (.%, (.%, (.%, (.%, (.%, (.%, (.%, (6.%, (6.%, (.%,.0....S.T.6.;... 0.; 0.6; 0.6; 0.;..6;.6;.].;. Study o. B Specificity (.%, (6.%, (6.%, (.%, (.%, (.%, (.%, (0.0%, (.%, (.0%, (.6%, (.%, (6.%, (6.%, (6.%, (6.%, (6.%, (6.%, (6.0%, (60.%, (0.%,.6).).).) 0.) 0.) 0.6) 0.0).).).).).).0) 6.) 6.) 6.) 6.).).).6) Study o. B Study o. PV neg (0%, (.%, (.0%, (.%, (.%, (.0%, (.0%, (.0%, (6 0%, (6.0%, (.%, (.%, (.6%, (.%, (.%, (.6%, (.%, (.0%, (.%,.0;.6.:....;..0.0;.;...;....0.; (.%,.0J (6.%,. PV pos (.%,.) (.%,.) (.%,.) (.%,.) B (.%,.) (6.%,.) (.0%,.) (6.%,.) (6.%,.0) (.%,.) (.%,.) (.%,.) (0.%,.) (.%,'.0) (.%,.) (.%,.) (.%,.) (.%,.) (6.%, 6.6) (.%,.) (.%,.) 'Study I indicates the male population, and study B indicates the female population. Downloaded from on February 0 October 6

3 cept or reject categories for specific cumulative weighted scores based on the previously published study. If the score was more than 0, the study was accepted. The model accepts the concept that screening is effective for eliminating the need to do a urine culture. If the score was 0 to 0, the study was conditionally accepted. The model accepts the concept that screening is effective for eliminating the need to do a urine culture but with reservations. If the score was below 0, the study was rejected. The model rejects the concept that screening is effective for eliminating the need to do a urine culture. nalysis of Studies From the Medical Literature Each of the urine studies selected was characterized by overall weighted score, study size, criterion for positive culture, and inclusion of and/or. dditionally, each urine study was grouped into one of the three scoring categories or au- thor acceptability, as previously described. ecommendations for Evaluation of Studies fter comparing the published scoring/ acceptability scoring model to the author (empirical) scoring/acceptability scores, we then synthesized a set of recommendations to be used as a guideline for assessing the acceptability of dipstick screening for urine culture. esults Table I summarizes the score for the performance characteristics for each of the urine studies incorporating and/or. Each study is ranked within category from the highest to the lowest. The identification number corresponding to the reference bibliography citation for each study is given first, followed by the percentage and weighted numerical score for each performance characteristic, respectively. Of interest, the trade- off be- Table II: Summary of Cumulative Overall Weighted Scores and anking of Individual Studies ( and/or ) eference* Overall Score Study Size ,000,000,,,000,00, B Overall ank Positive Culture Criterion >X"CFU/mL (voided urine) :X CFU/mL (catheterized urine) >X«CFU/mL >*CFU/mL(noncatheterized urine) > CFU/mL (catheterized urine) * and B represent two separate studies. 6 October Downloaded from on February 0 tween sensitivity and specificity for a laboratory test should be noted. For example, study is the least sensitive study (rank,.%) but is among the most specific (rank,6.%). tally of individual studies for sensitivity and PVneg showed the following count expressed as number of studies followed in parentheses by the range of sensitivity of PVneg: sensitivity,five(0% to 0%), ten (0% to 0%), three (0% to 0%), two (60% to 0%), one (0% to 60%); and PVneg, (0% to 0%), two (0% to 0%), one (60% to 0%). tally of individual studies for specificity and PVp0S revealed the following count: specificity, two (0% to 0%), six (0% to 0%), four (0% to 0%), eight (60% to 0%), one (0% to 60%); and PV^, two (0% to 0%), one (60% to 0%), four (0% to 60%), six (0% to 0%), four (0% to 0%), and four (0% to 0%). Table II summarizes the cumulative weighted score and overall ranking of each individual urine study incorporating and/or and contains data regarding study size, and/or, and the criterion adopted in each study for a positive urine culture. The majority of the studies (/, or %) included both and (the dipstick was considered positive if either and/or was positive). Table III summarizes the urine culture studies and whether there was agreement between the published scoring model and author acceptability. There was good agreement between the two scoring systems for the accept and reject categories, since the three urine studies with cumulative weighted scores exceeding 0 were accepted, whereas thefivestudies below 0 were rejected by both models. Disagreement between the two models was noted for the cumulative weighted scores between 0 and 0, a range in which we found empirically three accept,fiveconditional accept, andfivereject studies; we designated the 0 to 0 range as "equivocal." We reviewed these urine studies to determine whether there were common reasons for accepting or rejecting screening. uthors who accepted the concept that provides an effective screening mechanism for urine culture cited cost savings in technician time and reagent cost as the primary reason for acceptance. uthors who rejected the concept that provides an efficient screening for urine culture cited low sen-

4 Table III: Summary of Culture Studies and greement Between Published Scoring System and uthor cceptability of Screening for Culture Score, Study Score uthor ccept/eject' greement Between Published Scoring Model and uthor cceptability?t C C C C C >0 0-0 <0 ' indicates accept; C, conditional accept; and, reject. t indicates yes;, no. sitivity, ranging from.% for study to.% for study, as the primary reason for rejection. On the basis of comparison between the published scoring system and author acceptability, we synthesized the following set of recommendations that we believe provides a framework for classification of published as well as potential urine studies. ecommendations for Feasibility of Using Screening to Eliminate Culture If the score is greater than 0, accept the study. If the score is 0 to 0, the study is in the equivocal range. ccept or conditionally accept the study if cost savings is of prime importance. eject the study if sensitivities are unsatisfactorily low. If the score is less than 0, reject the study. C omment Many recent studies have focused on the diagnostic utility of dipstick testing as a screening technique for deciding whether a urine culture should or should not be done. Thus far, however, there has been no universal agreement regarding the reliability of this screening technique. Some authors advocate eliminating routine urine culture based on a negative, while others advocate continuing routine urine culture on all requests regardless of the test result."" Using the published scoring system, we found that ranking urine studies in the medical literature resulted in generally good agreement between the cumulative weighted score and the acceptability of the study. greement between the published scoring/acceptance model and the empirical scoring/acceptance model was good for cumulative scores more than 0 and less than 0. greement for scores between 0 and 0 was not good, with disagreement in approximately 0% of cases, which led us to label this scoring range as "equivocal." We found that the published scoring system used to assess the feasibility of eliminating urine microscopy could not be applied without modification to the problem of dipstick screening for urine culture. The altered set of recommendations are presented as a guide to provide a rational basis for accepting or rejecting dipstick screening for urine culture. Considerable variation in sensitivities were noted for urine studies in the literature, which may reflect the populations studied and the prevalence of urinary tract disease in these populations. Study illustrates the effect of varying populations on author acceptability of screening. This study consisted of two parts; in thefirstpart, the author accepted screening for urine culture in a male population (sensitivity of.%), but in the second part, the author rejected as a screening tool in a female population (sensitivity of 6.%). The effect of prevalence of urinary tract disease is illustrated by study (score of., equivocal range). screening was rejected by the authors who concluded that the sensitivity of.% was unacceptably low for their older male population (mean age of 6 years), in whom the prevalence of urinary tract disease may be high and in whom the morbidity of this disease is significant. Finally, we should emphasize that the screening technique for urine culture has been evaluated using the criterion of CFU/mL or more for the definition of significant bacteriuriaand that counts less than have been considered contaminants. ecently, Stamm and coworkers have found that approximately half of the symptomatic women with acute lower urinary tract infections due to coliform organisms had urine cultures of less than CFU/mL compared with the presence of the same bacteria from bladder specimens obtained by catheterization or suprapubic aspiration; these authors suggest that for maximum diagnostic sensitivity, the criterion of CFU/ ml or more be used (sensitivity, %; specificity, %). In light of these recent findings, the usefulness of screening based on the criterion of J CFU/mL Downloaded from on February 0 October 6

5 or more may be of limited value in the population of acutely dysuric women - D eferences. Lum G, Morrison C: Evaluation of dipstick urine studies using a scoring system based on test performance characteristics. m J Clin Pathol ;:-0.. Bartlett C, O'eill D, McLaughlin JC: Detection of bacteriuria by leukocyte esterase, nitrite, and the utomicrobic system. m J Clin Pathol ;:6-6.. Loo ST, Scottolini G, Luangphinith S, et al: screening strategy employing dipstick analysis and selective culture: n evaluation. m J Clin Pathol ;:6-6.. Malowany MS, Kittick J, Good M: Cost savings in microbiology urine screening: n alternative for routine culture. Lab Med ;:0-0.. Oneson, Groschel DHM: Leukocyte esterase activity and nitrite test as a rapid screen for significant bacteriuria. m J Clin Pathol ;:-. 6. Park CH, Hixon DL, Ferguson CB, et al: Bacteriuria screening by leukocyte esterase nitrite strip plus Gram stain. Vir Med ;:6-.. Perry JL, Matthews JS, Weesner DE: Evaluation of leukocyte esterase activity as a rapid screening technique for bacteriuria. J Clin Microbiol ;:-.. Pezzlo MT, Wetkowski M, Peterson EM, et al: Detection of bacteriuria and pyuria within two minutes. J Clin Microbiol ;:-.. Sawyer KP, Stone LL: Evaluation of leukocyte dipstick test used for screening urine cultures. J Clin Microbiol ;0:0-.. Smalley DL, Dittmann : Use of leukocyte esterase-nitrite activity as predictive assays of significant bacteriuria. J Clin Microbiol ::6-.. Wenk E, Dutta D, udert J, et al: Sediment microscopy, nitrituria, and leukocyte esterasuria as predictors of significant bacteriuria. J Clin Lab utomat ;:-.. Hughes JG, Synder J, Washington J : n evaluation of a leukocyte esterase/nitrite test strip and a bioluminescence assay for detection of bacteriuria. Diagn Microbiol Infect Dis ;:-.. Jones C, MacPherson DW, Stevens DL: Inability of the Chemstrip L compared with quantitative urine culture to predict significant bacteriuria. J Clin Microbiol 6;: Maksem J, Mulling D, Kwak S: Predicting urine culture results:the comparative values of nitrituria, leukocyturia, and microscopic bacteriuria. Lab Med ;:-.. Males BM, Bartholomew W, msterdam D: Leukocyte esterase-nitrite and bioluminescence assays as urine screens. J Clin Microbiol ;:-. 6. Murray P, Smith TB, McKinney TC: Clinical evaluation of three urine screening tests. J Clin Microbiol ;:6-0.. Pfaller M, Koontz FP: Laboratory evaluation of leukocyte esterase and nitrite tests for the detection of bacteriuria. J Clin Microbiol ;:0-.. Sewell DL, Burt SP, Gabbert J, et al: Evaluation of the Chemstrip as a screening test for urinalysis and urine culture in men. m J Clin Pathol ;:0-.. Warkentin DL, awling, Koppel WE, et al: Three techniques compared for detecting bacteriuria in symptomatic patients. Clin Chem ;: Wilkins EGL, atcliffe JG, oberts C: Leucocyte esterase-nitrite screening method for pyuria and bacteriuria. J Clin Pathol ;:-.. Stamm WE, Counts GW, unning K, et al: Diagnosis of coliform infection in acutely dysuric women. Engl J Med ;0:6-6.. Pollock HM: Laboratory techniques for detection of urinary tract infection and assessment of value. m J Med ;(lb):-. 00 October Downloaded from on February 0