Identification of Enterobacteriaceae by the automicrobic system: Enterobacteriaceae biochemical cards versus Enterobacteriaceae-plus biochemical cards.

Enterobacteriaceae Biochemical Cards (EBC) may be used in the AutoMicrobic system for identification of enteric bacilli. Recently, the card has been modified to permit identification of enteric and certain nonenteric bacilli. Also, minor modifications have been in the computer program used for interpretation of tests with the new cards (EBC+). The two types of cards (EBC and EBC+) were tested in parallel and found to be in agreement with 97% of 650 Enterobacteriaceae. Most of the discrepancies were resolved when selected strains were retested on 3 separate days. A lack of absolute reproducibility with either system was demonstrated and explained most of the initial discrepancies. Approximately 97% of the AutoMicrobic system identifications agreed with those obtained from standard reference methods, after equivocal AutoMicrobic system results (P less than 0.80) were excluded. Equivocal responses occurred with 4% of our EBC tests and 7% of our EBC+ tests; additional tests are needed before such strains can be identified with confidence.     

Detection of bacteriuria by leukocyte esterase, nitrite, and the automicrobic system

The predictive value for detection of significant bacteriuria was determined for use of the Chemstrip leukocyte esterase-nitrite dipstick (LN) and the Vitek Automicrobic System (AMS) using a conventional culture method (CM) as the reference procedure. The predictive values for positive and negative tests for detection of greater than or equal to 10(5) bacteria/mL for LN testing alone (2,782 specimens), AMS testing alone (729 specimens), and AMS testing of only LN positive specimens (253 specimens) were 29% and 97%, 67% and 99.5%, and 74.7% and 99.4%. The low predictive values for positive AMS tests were the result of discrepancies in enumeration in which counts of 10(4)/mL by CM often were classified as 10(5)/mL by AMS. Screening of specimens by LN reduces the cost of subsequent processing of LN positive specimens. Processing of LN positives by CM yielded the lowest total cost. Processing of LN positives by AMS resulted in the lowest labor cost.     

Reliability of early identifications obtained with Enterobacteriaceae-plus biochemical cards in the automicrobic system

The AutoMicrobic system (AMS) is capable of identifying most Enterobacteriaceae within 8 h and many glucose-nonfermenting, gram-negative bacilli after 13 h of incubation. Early preliminary results can be readily obtained from the computer as the tests incubate. Data with 1,023 bacterial isolates were reviewed to determine the relative accuracy of 4-, 6-, 8-, 10-, and 13-h identifications. All AutoMicrobic system identifications with probability (P) values of less than 0.80 were considered equivocal responses which needed supplementary tests before a final report could be issued. Analysis of our data suggests that early identifications of Morganella morganii, Acinetobacter sp., Yersinia spp., Salmonella spp. (other than Salmonella typhi), Shigella spp. (other than Shigella sonnei), Enterobacter agglomerans, Pseudomonas spp. (other than Pseudomonas aeruginosa or Pseudomonas maltophilia), Klebsiella spp. (other than Klebsiella pneumoniae or Klebsiella oxytoca), Citrobacter amalonauticus, Serratia liquefaciens, or Vibrio spp. Should be considered nonspecific responses, even when P greater than or equal to 0.80. Other identifications reported after 4 h were 96% accurate. At least half of our isolates (60% of our Enterobacteriaceae) could be identified reliably within 4 h, the remaining isolates required longer incubation.     

Evaluation of the automicrobic system gram-positive identification card for species identification of coagulase-negative staphylococci.

The AutoMicrobic system Gram-Positive Identification Card (Vitek Systems, Inc., Hazelwood, Mo.) was evaluated for identification of a group of 150 isolates of coagulase-negative staphylococci. Identifications obtained with the Gram-Positive Identification Card were compared with reference identifications derived from 15 conventional biochemical tests. The AutoMicrobic system correctly identified only 67.3% (101 of 150) of the test isolates. The greatest accuracy was achieved with Staphylococcus epidermidis isolates (95.7%), whereas Staphylococcus hominis isolates were least often correctly identified (26.7%).     

Evaluation of an automated, computerized system (automicrobic system) for Enterobacteriaceae identification.

The automated and computerized AutoMicrobic system (AMS; Vitek Systems, Inc., subsidiary of McDonnell Douglas, Hazelwood, Mo.) was evaluated as a means of identifying the Enterobacteriaceae. The Micro-ID system (General Diagnostics, Morris Plains, N.J.) and, when necessary, conventional tubed media were used for comparison. Identification by AMS and Micro-ID differed in only 12 of 1,528 isolates (0.8%). Disagreements occurred primarily with Enterobacter spp. Precision testing of the AMS showed only 1 of 72 tests (1.4%) deviating from the expected. The AMS was found to be an accurate and precise method for the identification of Enterobacteriaceae.     

Evaluation of Questor urine screening system for bacteriuria and pyuria.

AIMS--To evaluate the Questor automated bacteriuria and pyuria screening system; to compare its performance with that of a reference method; and to assess its usefulness in a routine clinical laboratory. METHODS--The Questor urine screening system was compared with a comprehensive regimen to detect urinary tract infection, using pour-plate viable counts to determine the numbers of bacteria present in urine samples, a wide range of other cultural methods, microscopic findings and clinical information. RESULTS--The optimal performance in detecting significant growths was a sensitivity of 93%, a specificity of 74%, a positive predictive value of 43% and a negative predictive value of 98%. The list price per test is 0.17 pounds and the capital cost of the system is 39,950 pounds. Questor can test 50 samples an hour and can be operated by one member of the laboratory staff, who is not required to make interpretative judgments--for example, a medical laboratory assistant. CONCLUSIONS--The sensitivity and specificity of the Questor was better than that obtained from other screening systems using the same protocol. The system was easy to use and is a useful addition to the methods available for screening for bacteriuria.     

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.     

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.     

Comparison of the automicrobic system, acridine orange-stained smears, and gram-stained smears in detecting bacteriuria

We compared the accuracy of the Gram-stained smear, the acridine orange-stained smear, and the AutoMicrobic system (AMS; Vitek Systems, Inc., Hazelwood, Mo.) in screening for bacteriuria, as detected by conventional cultures. For 1,024 clinical specimens, results with the acridine orange-stained smear and the Gram-stained smear were very similar. When read for the presence of one or more microorganisms or leukocytes per 20 oil immersion fields, both smears were highly sensitive (92.1 and 93.3%, respectively) and moderately specific (70.0 and 61.7%, respectively). Sensitivity was greater for specimens yielding greater than or equal to 10(5) CFU/ml (96.1 and 98.9%, respectively) than for those with 10(3) to 10(4) CFU/ml (81.4 and 78.0%, respectively). Preliminary classification based upon the tinctorial and morphological characteristics of the Gram-stained smear was compatible with culture results in nearly all cases. The accuracy of the Gram-stained smears was not influenced by special cleaning of the microscopic slides, or the level of expertise of the microscopist. For 715 specimens, the sensitivity of the AMS in detecting bacteriuria (91.5%) was very similar to that of the stained smears (92.1 and 95.7%, respectively), but the specificity was significantly higher (83.2% versus 42.6 and 70.0%). Detection of microorganisms by the AMS took an average of 6.3 +/- 3.0 h. These data suggest that the Gram-stained smear is easily interpreted, very sensitive, acceptably specific, and still the optimal rapid method for screening for bacteriuria in most clinical microbiology laboratories.     

Comparative evaluation of the API 20S system and the automicrobic system gram-positive identification card for species identification of streptococci.

Two commercial methods, the API 20S system (API; Analytab Products, Inc., Plainview, N.Y.) and the Gram-Positive Identification Card (GPI; Vitek Systems, Inc., Hazelwood, Mo.), were evaluated without additional tests for the identification of 241 streptococcus strains. Organisms included 60 beta-hemolytic strains, 36 group D strains, 26 Streptococcus pneumoniae strains, and 119 viridans streptococcus strains. API correctly identified to species 68.3% of beta-hemolytic strains, 86.1% of group D strains, 53.9% of S. pneumoniae strains, and 12.6% of viridans streptococci. This method provided excellent identification of group A and B and S. faecalis strains. Overall, API correctly identified 41.9% of strains to species, with 41.1% good likelihood but low selectivity, 15.8% incorrect, and 1.2% not identified. GPI correctly identified to species 58.3% of beta-hemolytic strains, 97.2% of group D strains, 80.8% of S. pneumoniae strains, and 57.2% of viridans streptococci. Group A, B, and D strains were all accurately identified by this system. Overall, GPI correctly identified to species 66.0% of strains, with 8.7% correct preliminary identification, 20.8% incorrect, and 4.6% not identified. Both methods represent a worthwhile advance in streptococcal identification. Neither system, however, can be recommended for species identification of the viridans group at this time.     

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.     

FiltraCheck-UTI, a rapid, disposable system for detection of bacteriuria.

The initial evaluation of the FiltraCheck-UTI bacteriuria detection system is described. The colorimetric test, which utilizes a disposable filter disk and a stable reagent system, does not require instrumentation. The test procedure is simple and may be performed in less than 1 min. Results obtained with the FiltraCheck-UTI system were compared with those obtained by conventional semiquantitative culturing. Of 1,198 urine specimens evaluated, 202 (16.9%) were determined to be significant positives at greater than or equal to 10(5) CFU/ml by the culture method. The sensitivity and specificity of the FiltraCheck-UTI system were 96.5 and 79.7%, respectively, and the negative predictive value was 99.1%. The high sensitivity, rapidity, simplicity, and unique disposable format of the FiltraCheck-UTI system offer significant advantages over other commonly used screening methods.     

Rapid screening of urine specimens for bacteriuria by the cellenium system

This study evaluated the performance of the Cellenium 160 US urine screening system in comparison to that of the semiquantitative culture method. The performance characteristics of the Cellenium system for all clinically significant uropathogens were 89.5% sensitivity, 94.4% specificity, 97.1% negative predictive value, and 81% positive predictive value.     

Use of the automicrobic and enteric-tek systems for identification of Enterobacteriaceae.

Studies were performed to evaluate the use of the Enteric-Tek (ET) and the AutoMicrobic system (AMS) for the identification of 201 Enterobacteriaceae freshly isolated from clinical specimens. All test systems were inoculated simultaneously from the same MacConkey agar plate. Organisms were also identified with conventional media. Identifications with the ET and AMS agreed with those made with conventional biochemicals 97% of the time. At a 95% confidence level, the ET was able to identify 75% of the isolates within 18 h without the aid of additional biochemical tests; the AMS identified 92% in 8 h. Technologist time needed for identifications made with the AMS was reduced approximately 57% compared with the ET. In all instances in which the AMS identification disagreed with the conventional, the ET identified the organism correctly. Similarly, organisms misidentified by the ET were correctly identified by the AMS. The data suggest that the AMS and ET identify clinical isolates with comparable accuracy; however, the AMS offers a significant savings in time.     

Evaluation of the automicrobic system for detection of resistance of Staphylococcus aureus to methicillin

The AutoMicrobic system (AMS) (Vitek System, Inc., Hazelwood, Mo.) was tested for its ability to determine oxacillin and gentamicin susceptibility of 98 known oxacillin-susceptible and 103 known oxacillin-resistant Staphylococcus aureus isolates. AMS and reference oxacillin susceptibility results were in agreement for all 95 (100%) oxacillin-susceptible isolates. In contrast, only 23 (22.3%) of the 103 known oxacillin-resistant isolates were correctly reported. For the known oxacillin-resistant isolates, 65 received AMS reports at 3 to 4 h, with only 9% being correct, whereas 38 were reported at 5 to 6 h, with 47% being correct. The reliability of AMS gentamicin susceptibility results was evaluated by testing the 198 S. aureus isolates in parallel with MIC-2000 broth dilution tests. AMS gentamicin susceptibility results were found to be reliable and essentially identical to MIC-2000 results. The possibility of improving AMS oxacillin resistance detection by using gentamicin resistance as a linked screening marker for oxacillin resistance was evaluated with data from the parallel AMS and MIC-2000 gentamicin susceptibility tests and from data accrued on recent clinical laboratory isolates. By these two approaches, respective sensitivities of 97 and 99.8%, and specificity of 72%, were found for detection of oxacillin-resistant isolates by using gentamicin resistance as a marker.