Reliability of latex agglutination tests for identification of Staphylococcus aureus resistant to oxacillin.

Commercial latex agglutination tests (LATs) for the simultaneous detection of clumping factor and protein A are gaining increased acceptance as a means of identifying Staphylococcus aureus. We evaluated two LATs (Accu-Staph; Carr-Scarborough, Decatur, Ga.; Staphaurex; Wellcome, Dartford, England) with particular emphasis on their ability to correctly identify oxacillin-resistant S. aureus. We tested 59 oxacillin-resistant S. aureus, 136 oxacillin-susceptible S. aureus, and 92 coagulase-negative staphylococcal strains with the two LATs and with thermonuclease, slide clumping factor, tube coagulase, and protein A hemagglutination tests. Clumping factor and protein A were present in 96.9 and 82.1% of our S. aureus strains, respectively. Accu-Staph correctly identified 92.8% and Staphaurex correctly identified 91.3% of S. aureus strains. No significant difference in LAT positivity rates, presence of clumping factor, or presence of protein A was found between oxacillin-resistant and -susceptible S. aureus. Overall, there were 31 false-negative LATs for 20 S. aureus strains, 14 with Accu-Staph and 17 with Staphaurex. Ninety-five percent of these strains possessed either clumping factor or protein A or both when these factors were determined independently. There were five false-positive LATs for four strains of coagulase-negative staphylococci (three Staphylococcus epidermidis and one Staphylococcus warneri), four with Accu-Staph and one with Staphaurex. Clumping factor was present in one S. warneri strain. Thus, the specificities of Accu-Staph, Staphaurex, and the clumping factor test were 95.6, 98.9, and 98.9%, respectively. Our results indicated that LATs identify oxacillin-resistant and -susceptible S. aureus equally well; however, they offer no greater sensitivity or specificity than the clumping factor test for identification of S. aureus.     

Comparison of five agglutination tests for identification of Staphylococcus aureus.

Various commercially produced agglutination kits are widely used for the identification of Staphylococcus aureus. These kits detect the presence of protein A and/or clumping factor on S. aureus. The literature has shown that methicillin-resistant S. aureus (MRSA) isolates which are deficient in both clumping factor and protein A may be misidentified. Two products, Slidex and Staphaurex Plus, utilize specific anti-S. aureus antibodies, potentially giving them greater sensitivity compared to products without these antibodies. We report a prospective study designed to compare the performance characteristics of Fastaph, Slidex, Staphaurex, Staphaurex Plus, Staphyloslide, and the tube coagulase test for the identification of staphylococcal isolates. All discrepant isolates were tested with the Gen-Probe AccuProbe S. aureus test and were identified to the species level with conventional reference biochemicals. A total of 1,193 isolates were tested, including 33 MRSA and 423 methicillin-sensitive S. aureus isolates. The sensitivities and specificities of the tests, respectively, were as follows: Fastaph, 99.1 and 98.9%; Slidex, 99.6 and 96.4%; Staphaurex, 98.9 and 99.9%; Staphaurex Plus, 99.6 and 93.9%; Staphyloslide, 99.1 and 98.9%; and tube coagulase, 99.3 and 100%. Sensitivity was excellent for all of the products tested. The specificities of Fastaph, Staphaurex, and Staphyloslide were excellent, while Staphaurex Plus and Slidex demonstrated less optimal results.     

Evaluation of the latex slide agglutination test for identification of Staphylococcus aureus.

This study evaluated the reliability of the latex slide agglutination test for identifying Staphylococcus aureus. A total of 806 clinical isolates of staphylococci were tested for latex agglutination, clumping factor, and free coagulase. Positive latex tests occurred in 98.3% of coagulase-positive strains, whereas 99.6% of coagulase-negative strains gave negative latex tests. It is concluded that in most instances, the latex slide agglutination test is a reliable method for identifying S. aureus in the clinical laboratory.     

Evaluation of a commercial latex agglutination test for identification of Staphylococcus aureus.

A new, commercially available latex agglutination test (SeroSTAT Staph; Scott Laboratories, Inc., Fiskeville, R.I.) was compared with the tube coagulase and slide coagulase tests as means for identifying Staphylococcus aureus. Of 160 clinical isolates of S. aureus, 159 (99.4%) yielded positive results with the latex agglutination test. Negative latex agglutination test results were obtained with 266 of 267 clinical isolates of Micrococcus spp. and staphylococcal species other than S. aureus (99.6%). The latex agglutination test was found to be a rapid, technically nondemanding method for identifying S. aureus. It was as accurate as the tube coagulase test and more accurate than the slide coagulase test.     

Rapid and reliable identification of Staphylococcus aureus by a latex agglutination test

A latex slide agglutination test detecting clumping factor and protein A simultaneously is recommended for rapid and reliable routine identification of Staphylococcus aureus. Strains (836) of staphylococci isolated from clinical specimens were examined, all S. aureus strains identified by conventional methods were correctly differentiated by the latex test, and no false-positive results occurred with other staphylococci. The reagent is easy to prepare since plasma is the coating material.     

Evaluation of three commercial agglutination tests for the identification of Staphylococcus aureus.

Three commercially available rapid slide agglutination tests for the identification of Staphylococcus aureus were evaluated with 354 recent clinical isolates (165 strains of S. aureus). The test results of two latex agglutination products, SeroSTAT Staph (Scott Laboratories, Inc.) and Staphylatex (American Micro Scan), and one hemagglutination product, Staphyloslide (BBL Microbiology Systems), were compared with the results of the tube coagulase test, which was read at 4 h (4-h tube coagulase test) and, if negative, again after overnight incubation at room temperature (24-h tube coagulase test). Discrepancies between agglutination and tube coagulase identifications were resolved by use of the thermonuclease, mannitol fermentation, and slide coagulase tests. All sensitivities, specificities, predictive values of a positive result, and predictive values of a negative result for the three agglutination tests were at least 98.8% and comparable with the 4-h tube coagulase test. Best results were obtained with the 24-h tube coagulase test, which yielded one false-negative and no false-positive tests. Agglutination identifications may be performed on organisms taken directly from a primary plate when sufficient growth is present. Kit agglutination procedures yield rapid and reliable identifications and are easy to perform. This study also demonstrates the usefulness of the 24-h tube coagulase test.     

Efficacies of rapid agglutination tests for identification of methicillin-resistant staphylococcal strains as Staphylococcus aureus.

Four commercially available rapid agglutination tests for the identification of Staphylococcus aureus were compared with the tube coagulase test for the identification of 300 methicillin-resistant isolates of staphylococci. Isolates tested included 207 methicillin-resistant S. aureus and 93 coagulase-negative staphylococci, collected from five medical centers. Strain variability was documented by phage typing and antimicrobial susceptibility patterns. Results of rapid identification tests ranged between 82 and 86% sensitivity, significantly poorer than the 98% sensitivity which the tube coagulase test provided.     

Comparison of a yellow latex reagent with other agglutination methods for the identification of Staphylococcus aureus.

A new commercial yellow latex agglutination reagent (Bacto-Staph) was compared with the slide and tube coagulase tests and three other commercial reagents for the identification of 283 Staphylococcus aureus and 54 non-S. aureus staphylococcal strains. Test sensitivities for the identification of S. aureus were as follows: tube coagulase, 99.6%; slide coagulase, 98.6%; Bacto-Staph, 99.6%; Staphylatex, 98.6%; Sero STAT Staph, 98.2%; and Staphyloslide, 97.5%. No false-positive reactions were observed with any of the commercial reagents.     

Direct identification of Staphylococcus aureus in blood culture fluid with a commercial latex agglutination test.

A commercial latex agglutination slide test (SeroSTAT Staph, Scott Laboratories, Inc., Fiskeville, R.I.) accurately identified Staphylococcus aureus when applied directly to blood culture fluid containing staphylococci. This latex agglutination test exhibited 100% accuracy when 30 seeded aerobic and anaerobic radiometric blood cultures (15 strains of S. aureus, 15 strains of other staphylococcal species) were tested blindly. In 36 actual clinical specimens yielding 16 isolates of S. aureus and 20 isolates of Staphylococcus epidermidis, 94.4% accuracy was achieved. The latex agglutination test provided positive test results before objective criteria of blood culture positivity such as radiometric growth indices and Gram stains became positive.     

Evaluation of penicillin binding protein 2a latex agglutination assay for identification of methicillin-resistant Staphylococcus aureus directly from blood cultures

The penicillin binding protein 2a (PBP2a) latex agglutination test using a blood culture pellet was compared to the oxacillin screen agar method using isolated colonies. For blood cultures positive for Staphylococcus aureus (n = 70), the direct PBP2a test was 18% sensitive and 100% specific. The PBP2a test shows poor sensitivity when used directly with positive blood cultures.     

Evaluation of various rapid agglutination methods for the identification of Staphylococcus aureus

A latex agglutination test (SeroSTAT Staph; Scott Laboratories, Fiskeville, R.I.) and two hemagglutination tests (Staphyloslide; BBL Microbiology Systems, Cockeysville, Md.; and Hemastaph; Remel, Lenexa, Kans.) were compared with the slide coagulase (SC) and tube coagulase (TC) tests at room temperature (22 to 25 degrees C) and at 37 degrees C for the rapid identification of Staphylococcus aureus. A total of 380 clinical strains of staphylococci were tested. The TC test performed at room temperature yielded the largest number of TC-positive results (n = 239), and based on this observation 239 organisms were classified as S. aureus and 141 were classified as non-S. aureus. The SC, TC (37 degrees C), SeroSTAT Staph, Staphyloslide, and Hemastaph tests correctly identified 210 (87.9%), 221 (92.5%), 238 (99.6%), 239 (100%), and 236 (98.7%) of the S. aureus isolates, respectively. Of the S. aureus isolates that were TC positive at room temperature 68% required 24 h of incubation before coagulase production was detected. There was one false-negative SeroSTAT Staph result and one false-negative Hemastaph result. The Staphyloslide test yielded two noninterpretable results (both organisms were later confirmed as non-S. aureus), whereas there were six noninterpretable results recorded with the Hemastaph test (four organisms were classified as non-S. aureus, and two were classified as S. aureus). The SeroSTAT Staph, Staphyloslide, and Hemastaph tests were all more sensitive than the conventional SC and TC (37 degrees C) tests and were considerably more rapid than the TC test at either temperature.     

Novel latex agglutination method with chicken anti-protein A for detection of Staphylococcus aureus infections.

A latex agglutination assay for the detection of protein A-secreting Staphylococcus aureus strains or strains with protein A in the cell wall is described. The assay utilizes latex particles coated with chicken anti-protein A antibodies. Chicken antibodies do not react with protein G-producing streptococci or rheumatoid factor, thus avoiding false-positive reactions.     

Fundamental study on latex reagents for agglutination tests

Competitive adsorption of Fab and Fc fragments on to particles revealed that the main driving forces for the adsorption of Fab and Fc fragments are ionic and hydrophobic forces, respectively. Latex particles were sensitized with antihuman C-reactive protein-antibody under a condition where ionic binding force was suppressed, and hence antibody was supposed to attach to the particles predominantly at the Fc site. The resulting latex indicated a high efficiency for the determination of C-reactive protein. Among the latexes used, a partially hydrolysed styrene-acrylamide copolymer latex was the best with respect to test efficiency and storage stability.     

Screening for methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci: an evaluation of three selective media and Mastalex-MRSA latex agglutination

Laboratory confirmation of MRSA is important for the implementation of infection control; conventional screening culture methods take up to five days for confirmation. The purpose of this study is to ascertain the efficiency of three selective media for growth of methicillin-resistant Staphylococcus aureus (MRSA) before and after enrichment in salt broth, and to evaluate the Mastalex-MRSA latex agglutination kit for detection of methicillin resistance. Screening swabs were collected from 63 patients, yielding 125 S. aureus isolates and 40 coagulase-negative staphylococcus (CNS) isolates. Selective media used were mannitol salt agar (MSA), Baird-Parker agar with ciprofloxacin (BPC) and bromocresol purple (BCPA). Polymerase chain reaction (PCR) for mecA gene detection was used as the reference standard for evaluation of the Mastalex-MRSA assay, which was also evaluated on colonies of S. aureus from the selective media. No significant difference was found in efficiency of MRSA isolation among the selective media. Pre-enrichment in the salt broth did not enhance isolation of MRSA. Methicillin-sensitive S. aureus and CNS were significantly inhibited in all selective media (P<0.05). Only BPC significantly selected out methicillin-resistant CNS (P<0.01). Mastalex-MRSA was 97% specific and sensitive for the detection of MRSA. It was 65% sensitive and 100% specific in detecting methicillin resistance in CNS. In conclusion, all selective media performed equally well (MRSA isolation rate of approximately 80%). Mastalex-MRSA provided rapid and reliable detection of MRSA from selective media, reducing the time required for confirmation of this organism.     

Comparative evaluation of a latex test for the identification ofStaphylococcus aureus

A rapid latex agglutination test, Staphaurex, was tested for its ability to identifyStaphylococcus aureus using 72 reference strains and 785 clinical isolates of the familyMicrococcaceae. All reference strains ofStaphylococcus aureus were Staphaurex-positive. Non-Staphylococcus aureus reference strains were negative. Using clinical strains, the results of the Staphaurex test were compared with the results of other tests commonly used to identifyStaphylococcus aureus. A total of 393 clinical isolates were classified asStaphylococcus aureus. The Staphaurex, slide coagulase, tube coagulase/human plasma and tube coagulase/rabbit plasma tests correctly identified 98%, 93.6%, 93.6% and 97.5% of theStaphylococcus aureus strains, respectively. The performance of the Staphaurex test, in terms of sensitivity and specificity, was significantly better than the slide coagulase test. It was as sensitive and almost as specific as the tube coagulase rabbit test and more sensitive than the tube coagulase human test.     

Evaluation of new agglutination test for identification of oxacillin-susceptible and oxacillin-resistant Staphylococcus aureus.

A new agglutination test (Monostaph +; Bionor, Skien, Norway) has been developed. This new agglutination test has been compared with two other agglutination tests for the identification of 128 isolates of Staphylococcus aureus and 82 coagulase-negative staphylococci. The sensitivities of both Monostaph + and Pastorex Staph-Plus were excellent (98.7 and 97.4%, respectively) in detection of oxacillin-resistant Staphylococcus aureus. The specificity was 96.4% (two Staphylococcus epidermidis isolates and one Staphylococcus hominis isolate were false positive).     

Practical therapeutic application of the oxoid PBP2' latex agglutination test for the rapid identification of methicillin-resistant Staphylococcus aureus in blood cultures

The Oxoid PBP2' latex agglutination test (OLA; Oxoid, Basingstoke, England) was evaluated in a controlled prospective study examining Staphylococcus aureus from 25 positive blood cultures. Subcultures of positive blood cultures with coagulase-positive, gram-positive cocci in clusters were batched, and the OLA was performed at the end of the working day, once growth was seen on the plate. Results were sent to the infectious disease pharmacist for therapy evaluation, and the 24-hour minimum inhibitory concentration (MIC) was confirmed the next day. Blood culture OLA results correlated 100% with oxacillin MIC results for the patient, and results were available in as little as 3 hours after the blood culture was positive. The mean time difference between the OLA and MIC reports was 19.4 hours. This test allowed same-day resistance marker reporting and was easily incorporated into the work flow of the clinical laboratory.     

Problems with rapid agglutination methods for identification of Staphylococcus aureus when Staphylococcus saprophyticus is being tested.

Six rapid agglutination tests for identification of Staphylococcus aureus were evaluated by using 62 strains of S. aureus, 63 strains of S. saprophyticus, and 67 strains of other coagulase-negative staphylococci. S. saprophyticus was responsible for 19 of 26 false-positive results and 20 uninterpretable reactions. Thus, urinary staphylococcal isolates that are positive by rapid agglutination tests may require other confirmatory tests for the identification of possible S. saprophyticus.