Screening for Asymptomatic Bacteriuria in Pregnancy: Urinalysis Vs Urine Culture Abdulrazak Abyad, M D, M PH Tucson, a Rizona
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Screening for Asymptomatic Bacteriuria in Pregnancy: Urinalysis vs Urine Culture Abdulrazak Abyad, M D, M PH Tucson, A rizona Background. Asymptomatic bacteriuria is common dur organism with a bacteria level o f at least 1000 organ ing pregnancy. Its average prevalence is 6%. It is an isms per milliliter. In the prediction of a positive cul important risk factor for acute pyelonephritis, hyper ture, the microscopic findings o f five or more leuko tension, preeclampsia, fetal wastage, low birthweight, cytes per high-power field (HPF) showed a sensitivity and prematurity. This study was performed to deter o f 94.4% and a specificity o f 95.0% . mine the usefulness of urine microscopy as a substitute Conclusions. Physicians should test the urine of for doing a screening urine culture. all prenatal patients at their first visit and send to the M ethods. The medical records o f all first trimester laboratory only those specimens with 5 or more obstetric visits from 1984 to 1990 were reviewed at a leukocytes per HPF. Using this method, unnecessary major university. The results o f 888 screening urinaly screening urine cultures will be substantially ses were recorded and compared with those of subse reduced. quent urine cultures. K ey words. Bacteriuria; pregnancy; urine; culture; R esults. Fifty-four cultures had growth o f a single microscopy. / Fam Pract 1991; 33:471-474. Pregnancy substantially alters the function o f the urinary nancy can reduce the incidence of pyelonephritis and tract1 and increases the risk o f infection, potentially caus possibly the incidence of low birthweight babies.13- 16 ing maternal and fetal morbidity. Asymptomatic bacteri Thus, screening for bacteriuria has become a standard uria often precedes symptomatic urinary tract infections component of the prenatal visit.7 Some clinicians, how in pregnant patients.2 The average prevalence of bacteri ever, do not screen their pregnant patients. Their reluc uria is 6%, with a range of 2.3% to 17.5% reported in tance is due to the false belief that the incidence o f the literature.3-7 When bacteriuria remains untreated bacteriuria is low,3-6 or that screening by urine culture is during pregnancy, 30% to 40% o f patients will progress overly expensive. to symptomatic pyelonephritis,1 which is the most fre In today’s cost-conscious environment, it is sensible quent cause o f antenatal hospital admissions.8 to limit the use of urine cultures and to substitute less Both hypertension and preeclampsia have been re expensive tests when feasible. Urine analysis is a possible ported to occur more frequendy in pregnant women substitute for urine culture.17 In fact, clinical studies with asymptomatic bacteriuria.9’10 Long-term sequelae, show that pyuria is present in 80% to 95% o f cases o f such as chronic pyelonephritis and renal failure, may proven bacteriuria (including those with bacterial colony follow bacteriuria.11 Other studies have suggested that counts o f less than 10,000/mL). Other studies, however, bacteriuria is associated with a high risk o f fetal wastage, contest the value o f pyuria in predicting bacteriuria on dorsal midline fusion defects,11 low birthweight, and the basis o f an inadequate specificity o f 50% to 76% .18’19 prematurity.12 The most accurate method o f estimating the number Multiple authors have shown that the treatment of o f white cells in the urine (pyuria) is to determine the asymptomatic bacteriuria in the first trimester o f preg- leukocyte excretion rate.18’19 This method is not practi cal, however, for routine office use.18’19 Alternative meth ods include enumeration of leukocytes using a counting Subm itted, revised, July 8, 1991. chamber, which has been found to correlate well with the From the Department ofFam ity M edicine, American University o f Beirut. Requests for leukocyte excretion rate.19-22 reprints should be addressed to A bdulrazak Abyad, AID, AtPH, Department o f Family The estimation of white blood count (WBC) per and Community M edicine, University o f Arizona, 1821 East Elm St, Tucson, AZ 85719. high-power field (HPF) is simple, fast, and the most © 1991 Appleton & Lange ISSN 0094-3509 The Journal o f Family Practice, Vol. 33, No. 5, 1991 471 Asymptomatic Bacteriuria in Pregnancy Abyad commonly used procedure to quantify pyuria in clinical Table 1. Number o f Leukocytes per HPF and Results of practice. According to previous studies, however, this Concurrent Urine Culture method lacks precision.23 Also, considerable differences Leukocyte Result of Urine Culture exist in the recommended pathological threshold values per HPF Positive* Negative Total for pyuria, possibly because o f uncontrolled variables and None 0 555 technical error.24^26 555 1-4 3 240 243 The goal o f this study was to evaluate the accuracy in 5 -8 12 27 39 predicting the presence or absence of significant asymp > 8 39 12 51 Total 54 834 888 tomatic bacteriuria based on the results o f pyuria levels *Positive culture is defined as >1000 organisms/mL o f a single organism. determined by simple microscopy. H PF denotes high-power field. would appear as a 45° line through the origin. The Materials and Methods typical R O C curve is a smooth, concave curve. This study was performed in the Family Medical Center at the American University of Beirut, a model residency- faculty practice with 3000 registered patients and 5000 patient visits each year. The charts o f all patients in their Results first trimester of pregnancy who visited the Family Med The mean age o f the patients included in the study was icine Center between 1984 and 1990 were reviewed. 26.43 years with a standard deviation o f 6 years and a Only patients who were asymptomatic and had no coex range o f 15 to 4 4 years. isting medical problems at the time of presentation were Using a level of bacteriuria of 1000 organisms per included in the study. milliliter as the cutoff point, cultures of 54 (6.08%) of The results of screening urinalyses were recorded the 888 clean, midstream urine specimens were positive. and compared with the results obtained on subsequent Positive culture at a colony count of >100,000/mL was urine cultures. All of the specimens were obtained by the 48 (5.4% ) for the increasing levels o f bacteriuria. Table 1 “clean-catch” method on the first obstetric visit. Any shows the results determined by leukocyte count per complications that had developed and had been docu HPF vs urine culture results. Table 2 shows the results of mented in the patient’s chart at the time of the antenatal the assessment o f the selected levels o f leukocyte number follow-up, such as pyelonephritis or preterm labor, were per H PF. The sensitivity and specificity o f leukocyte level noted. in the prediction of a positive culture and the predictive For the purpose of this study, asymptomatic bacte riuria is defined as the presence of > 1000 colonies of values o f positive and negative tests are shown. bacteria per milliliter of urine from a specimen obtained The R O C curve o f leukocyte count as a predictor of by the clean-catch, midstream collection method. The asymptomatic bacteriuria is shown in Figure 1. Using a microscopic examination was performed on a urine sam leukocyte count of > 8 as the cutoff point, the sensitivity ple centrifuged at 2000 rpm for approximately 5 min was 72%, while the false-positive rate fell to 1%. Figure utes. Sensitivity, specificity, and positive-predictive value 1 illustrates that as the sensitivity increases, the false were calculated to assess the utility o f using the W BC to positive rate increases. When sensitivity was 100%, the predict the presence or absence of bacteriuria. false-positive rate was 33% . Receiver operating characteristic (ROC) curves of leukocyte count by microscopy of urinary sediment were constructed by establishing incremental cutoff points. Table 2. Prediction o f Bacteriuria (> 1000 organisms/mL) by The selected cutoff points for pyuria were >8 WBC/ Level of Leukocyte Count for Centrifuged Urinary Sediment (n = 888) HPF, >5 WBC/HPF, and >1 WBC/HPF. An ROC curve demonstrates the relationship be Leukocyte Positive- Negative- tween sensitivity and specificity for various cutoff points Count Predictive Predictive (WBC/HPF) Sensitivity Specificity Value on a screening test. Lowering the cutoff point to increase Value sensitivity inevitably results in increasing the false-posi > 8 72.2 98.6 76.5 98.2 a 5 94.4 95.3 56.7 99.6 tive rate, thus decreasing the specificity. Increasing the al 100 66.6 16.2 100 cutoff point, however, lowers the false-positive rate, thus W BC denotes white blood count; HBF, high-power field. decreasing the sensitivity. A test with no diagnostic value 472 The Journal o f Family Practice, Vol. 33, No. 5, 1991 Asymptomatic Bacteriuria in Pregnancy Abyad cutoff point is to minimize the sum of both false-positive and false-negative test results. Receiver operating characteristic curves also provide information that is helpful in establishing cutoff points for diagnostic testing. This process is tied to cost-effec tiveness. There are benefits in establishing early diagnosis of asymptomatic bacteriuria in pregnancy. The costs of false-positive test results are not high and there are no hazardous, unpleasant, expensive, or invasive diagnostic or treatment modalities. The cost o f a false-negative result, however, is high. Therefore, one would select a low cutoff leukocyte count in order not to miss many pregnant women with asymptomatic bacteriuria. To avoid missing even a single urinary tract infec tion, the recommendation would be to screen all patients with microscopy and to send those with > 1 WBC/HPF 1-Specificity (False-Positive Rate) for urine culture.