Evaluation of the modified Bac-T-Screen and FiltraCheck-UTI urine screening systems for detection of clinically significant bacteriuria

Previous evaluations of the Bac-T-Screen system (Vitek Systems, Inc., Hazelwood, Mo.) demonstrated excellent sensitivity with specimens from patients with clinically significant bacteriuria (including infections with small numbers of uropathogens) but poor specificity with specimens from noninfected patients. In the study reported here, the sensitivity and specificity of the Bac-T-Screen system with a modified decolorizing reagent were evaluated. A manual filtration system, FiltraCheck-UTI (Vitek Systems), for screening urine specimens, Gram stains of mixed urine specimens, and quantitative cultures were also evaluated. The test sensitivity for clinically significant bacteriuria was greater than 96% with the original Bac-T-Screen system as well as the modified system and the manual system. In comparison, the sensitivities of the Gram stains and quantitative cultures (greater than or equal to 10(5) CFU/ml) were 82 and 77%, respectively. Of the 375 patients classified as noninfected by clinical parameters, 34% had positive screening tests with the original Bac-T-Screen system, as compared with 13 and 11% with the modified Bac-T-Screen and FiltraCheck-UTI systems, respectively. Thus, the modified Bac-T-Screen system and the manual FiltraCheck-UTI system have sensitives comparable to that of the original Bac-T-Screen system and markedly improved test specificities.     

Evaluation of six screening methods for detecting significant bacteriuria.

Six screening methods for the successful detection of significant bacteriuria--electrical impedance (Malthus), automated acridine-orange staining (Autotrak), particle counting (Ramus), bioluminescence, nitrite and leucocyte test strip (BM Nephur), and microscopy--were evaluated. All had excellent predictive values for a negative result (97%-100%) but were less accurate in predicting a positive result (31%-83%). All methods had high sensitivities (83%-100%) but lower levels of specificity (68%-79%). Bioluminescence was the method with the highest specificity (79%) and the lowest rate of false positive results (15%). It would be inappropriate to decide on treatment and management on the basis of the positive results achieved with any of the methods evaluated, but all methods tested could be used for screening out negative results.     

Comparative evaluation of the Limulus assay and the direct Gram stain for detection of significant bacteriuria.

A double-blind study comparing the Limulus in-vitro endotoxin assay with the direct Gram stain of uncentrifuged urine for detection of significant bacteriuria was performed. One-thousand seventy-seven urine specimens were examined by the two methods and the results compared with results of quantitative urine cultures. Two hundred three samples produced growth of greater than 10-5 organisms per ml. urine. The Limulus assay detected 86.2% of these specimens, and 98.8% of urines that contained greater than 10-5 Gram-negative bacilli per ml. The Gram stain procedure detected only 69.5% of urines containing greater than 10-5 organisms per ml. and 74.5% of specimens with greater than 10-5 Gram-negative bacteria per ml. urine. The Limulus assay demonstrated both greater sensitivity and greater specificity than the Gram stain procedure. Moreover, the Limulus test is much less susceptible to errors of interpretation than methods involving microscopy.     

Rapid screening for significant bacteriuria using a Coulter Counter.

A Coulter Counter was evaluated for detecting significant bacteriuria. Simple urinary particle counts showed agreement in 883 of the 956 (92.4%) urine specimens examined compared to a standard culture procedure. They also provided a semiquantitative estimate of bacterial numbers in specimens diagnosed as infected by culture. Sample preparation before counting was minimal, and results were obtained within two to four minutes.     

Method for the detection of significant bacteriuria in large groups of patients

A measured area of blotting paper was used as a vehicle for transferring a constant aliquot of urine on to the surface of a culture medium. The number of bacterial colonies growing in the inoculum area of the medium corresponded to the number of organisms in the urine. One thousand, two hundred and seventy-five urines from women attending an ante-natal clinic were tested by this method, and the results compared with the results of quantitative bacterial counting and of triphenyl tetrazolium chloride (T.T.C.) tests. The method was found to be a simple, cheap, and reliable way of screening urine specimens for significant bacteriuria.     

Clinical evaluation of three rapid methods for the detection of significant bacteriuria.

Random urine specimens (848) were screened for significant bacteriuria by using the 30-min Lumac (3M, St. Paul, Minn.), the 2-min Bac-T-Screen (Marion Laboratories, Inc., Kansas City, Mo.), and the 13-h AutoMicrobic system (AMS) urine identification card (Vitek Systems, Inc., Hazelwood, Mo.). MacConkey and 5% sheep blood agar plates were inoculated with a 10(-4) dilution of urine and used for the reference method. Bac-T-Screen results were uninterpretable for 9.1% of the specimens owing to either urine sample pigmentation (5.3%) or clogging of the filter (3.8%). Screen-negative urine specimens made up 49.6, 57.2, and 48.5% of the total number of specimens evaluated with AMS, Lumac, and Bac-T-Screen, respectively. False-positive results with Lumac and Bac-T-Screen were 20.6 and 22.3%, respectively. False-negative results for cultures with greater than or equal to 10(4) CFU/ml were 22.0% with AMS, 29.4% with Lumac, and 25.5% with Bac-T-Screen, and false-negative results for cultures with greater than or equal to 10(5) CFU/ml were 29.6% with AMS, 9.9% with Lumac, and 7.0% with Bac-T-Screen. For each system, greater than 70% of false-negatives at greater than or equal to 10(5) CFU/ml consisted of mixed or pure cultures of common contaminants. With any of these screening methods, a clinically significant isolate at greater than or equal to 10(5) CFU/ml would rarely be missed (less than or equal to 1.7% for all systems). A cost-effective and rapid approach to urine microbiology could consist of screening out negative specimens by either Lumac or Bac-T-Screen and processing only screen-positive specimens by the AMS.     

Simplified microscopy for rapid detection of significant bacteriuria in random urine specimens.

Simplified urine microscopy, nitrite testing, and dipstick culture were compared with urine loop streak culture colony counts in 219 random voided specimens to determine the accuracy of the three rapid screening techniques. Nitrite testing resulted in 65% false negative results, which could not be significantly improved by incubation at 37 degrees C but which could be improved by adding nitrate substrate before incubation. Dipstick culture could not be quantitated until after 18 h of incubation. A new, simplified microscopy technique, using unspun, unstained urine, resulted in 4% false negative results and 4% false positive results in specimens containing over 10(5) organisms per ml and was the best method Centrifuges, Gram staining reagents, and counting chambers are not necessary for accurate microscopic screening of random urine specimens for the presence of bacteriuria by this technique, and the results are immediately available.     

Evaluation of a nitrite test kit (Stat-test) for the detection of significant bacteriuria

A commercially marketed nitrite test kit (Stat-test) for the rapid detection of significant bacteriuria has been evaluated and found to have failed to detect 50% of all cases of significant bacteriuria and 46% of significant bacteriuria associated with nitrate-reducing organisms. The number of false positive results are negligible. The Stat-test, used in its present form, cannot be recommended as a satisfactory screening test for significant bacteriuria.     

Clinical laboratory evaluation of a bacteriuria detection device for urine screening

The BAC-T-SCREEN (BTS) (Marion Laboratories, Inc., Kansas City, MO) is a 2 1/2-minute urine screen designed to detect culture-negative specimens. A total of 1,609 urine specimens were tested by the BTS, and results were compared with quantitative culture methods. One hundred and forty-eight (9.2%) specimens were not screened successfully by the BTS because they contained interfering pigments or clogged the test filters. A total of 1,461 specimens were tested successfully. The sensitivity, specificity, positive predictive value, and negative predictive value for specimens containing greater than or equal to 10(5) CFU/mL were 98.0, 72.2, 57.3, and 99.0%, respectively. These values for specimens containing greater than or equal to 10(4) CFU/mL were 93.2, 77.2, 69.2, and 95.5%, respectively.     

Evaluation of the MS-2 urine screening method for detection of bacteriuria.

A collaborative evaluation of the MS-2 urine screening system indicated that the automated method could successfully identify at least 84% of urine specimens that contained greater than or equal to 10(5) colony-forming units per ml. Most specimens not detected as positive by the system contained primarily diphtheroids and yeasts. Excluding those specimens which contained diphtheroids and yeasts, the MS-2 system correctly identified as positive 94.8% of specimens that contained greater than or equal to 10(5) colony-forming units per ml and as negative 96.1% of specimens that contained less than 10(4) colony-forming units per ml. The automated system is an attractive method for the screening of urine specimens for significant bacteriuria.     

Laboratory evaluation of leukocyte esterase and nitrite tests for the detection of bacteriuria.

We compared the sensitivity, specificity, and predictive values of the 1-min leukocyte esterase test and the test for urinary nitrite alone and in combination as screening tests for bacteriuria in over 5,000 clinical urine specimens. The leukocyte esterase-nitrite combination had a sensitivity of 79.2%, a specificity of 81%, and a negative predictive value of a negative test of 94.5% for specimens with greater than or equal to 10(5) CFU/ml. Although the sensitivity of this test was too low to allow its use as the only screening test for bacteriuria, it may serve as a useful adjunct to culturing and other urine-processing systems in the microbiology laboratory.