Isobel Rutherford

Aberrant Histoplasma capsulatum confirmation of identity by a chemiluminescence-labeled DNA probe

A cottony, light tan, filamentous fungus with pear-shaped microconidia and lacking tuberculated macroconidia was isolated from a bronchial lavage specimen. Subculture on several media at 37 degrees C failed to convert the fungus to a yeast form after several weeks; attempts at in vivo conversion in mice were also unsuccessful. Sera obtained several months apart showed M bands with Histoplasma capsulatum (HC) antigen by immunodiffusion and an increase in complement fixation titers with mycelial and yeast phase antigens of HC. Parallel identity was obtained on two occasions with exoantigen culture confirmation reagents for HC from Immuno-Mycologics as well as one of identity with Nolan reagents. Extracts from four Chrysosporium spp. strains had no identity reactions with HC with either kit. The fungus was identified as HC by the Accuprobe Histoplasma chemiluminescence-labeled DNA probe directed at ribosomal RNA, whereas all four Chrysosporium spp. isolates tested negative. DNA probes are a fast and accurate method to confirm the identity of aberrant fungal isolates.

Comparison of vidas Clostridium difficile toxin-A assay and premier C. difficile toxin-A assay to Cytotoxin-B tissue culture assay for the detection of toxins of C. difficile

Damage to the intestinal mucosa by Clostridium difficile (CD) is toxin mediated. Two enzyme immunoassays (EIAs) for toxin-A detection, the automated Vitek immunodiagnostic assay system CDA (Vidas CDA), and the Premier toxin A (Premier) were tested for their ability to detect toxin A in 301 stool samples and compared with an in-house tissue culture assay for toxin B (TCA). Of these 301 samples, 49 were TCA positive and 252 were TCA negative. Agreement between Vidas CDA and TCA on the initial run was 85% (255 of 301) and increased to 94% (278 of 296) when discordant samples were retested from available frozen specimens. Corresponding levels of agreement for Premier were 91% (272 of 301) and 98% (284 of 288), respectively. If tissue culture positivity at any titer was used as the sole criterion for positivity of the specimen, agreement with positive TCA before and after repeat testing was 57% (26 of 49) and 74% (34 of 46) for Vidas CDA and 65% (32 of 49) and 95% (36 of 38) for Premier. Agreement with negative TCA titers was good: 90% for Vidas CDA and 95% for Premier, and 98% for Vidas CDA and 99% for Premier after repeat testing. Predictive values positive and negative after repeat testing were, respectively, 88% and 96% for Vidas CDA, and 95% and 99% for Premier. Results for the automated and manual EIA methods for detection of C. difficile toxin A were obtained in 2.5 h as compared with 36-48 h for tissue culture.

Comparison of two commercial enzyme-linked immunosorbent assays and fluorescent antibody to membrane antigen test for immune status to Varicella Zoster virus

To determine the immune status to Varicella Zoster virus (VZV), a simple rapid method such as an enzyme immunosorbent assay (EIA) is needed as an alternative to the VZV fluorescent antibody to membrane antigen test (VZV-FAMA). With the VZV-FAMa (Viran Clinical Diagnostics, Stevensville, MI), a titer of ≥ 1:2 suggests previous exposure to VZV (positive) whereas < 1:2 suggests susceptibility to VZV (negative). Two EIA kits, Varicelisa® and Varicella STAT® (M.A. Bioproducts, Walkersville, MD) were compared with VZV-FAMA and with each other. The Varicelisa is a 3 h test whereas the Varicella STAT® is a 1 h test due to substrate differences. There were 78 positive and 24 negative sera by VZV-FAMA from 102 patients. Varicelisa® agreed with VZV-FAMA in 94 patients (92%); 71 positive and 23 negative. For Varicella STAT®, 88 sera (86·2%) agreed with VZV-FAMA assay; 65 positive, 23 negative. The sensitivity and specificity of the Varicelisa® were 91% (7178) and 95·8% (2324); of Varicella STAT® 83·3% (6578) and 95·8% (2324) respectively.