The Utility of Microbiological Studies in Diagnosis and Management of Suspected Dermatological Infection

Microbiological studies are often performed concurrently with histologic study of skin biopsy specimens to evaluate suspected infection. These studies may require an additional biopsy, with possible patient discomfort, procedure-related risk, and increased laboratory time and cost.^1,2 Despite the commonality of this practice, data on the utility of tissue microbiological studies are limited. In this study, we evaluate the yield of skin microbiology studies and how often they influence care in the general dermatology population.

We searched the Partners Healthcare registry for patients for whom both pathology and microbiology studies were ordered on skin biopsy specimens by dermatologists at Brigham & Women’s Hospital and Massachusetts General Hospital from 2000 to 2015. Medical record review was conducted on a random sample of 150 patients with complete clinical documentation who met inclusion criteria.

Each set of microbiology results was defined as true-positive, false-positive, true-negative, or false-negative based on the documented final clinical interpretation. We defined false-negative as cases where clinical suspicion overruled negative data and false-positive as cases where findings showed clinically nonrelevant organisms such as skin commensals. Microbiology sets were categorized based on positive result interpretation (eg, a hypothetical set with true-negative bacterial culture, false-positive Gram stain, true-positive acid-fast bacillus culture, and true-negative fungal stain and culture would be categorized as true-positive). The study was approved by the Partners institutional review board. Written informed consent was not required because this was a retrospective study.

Of 150 patients, 138 (92%) were seen in outpatient dermatology clinics and 65 (43.3%) did not have a documented history of immunosuppression (Table 1). Infection was listed among the top differential diagnoses in 105 (70%) of patients (Table 2). The most common microbiological studies were bacterial culture (144 patients, 96%), Gram stain (123 patients, 82%), and fungal culture (123 patients, 82%).

Gram stains were positive in 15 microbiology sets (12.2%), fungal stains were positive in 0 (0%), and acid-fast bacilli stains were positive in 2 (1.8%) cases. Bacterial cultures were positive in 72 (50%), fungal cultures were positive in 6 (4.9%), and acid-fast bacillus cultures were positive in 5 (4.6%). Microbiological studies were 70.7% (106 of 150) concordant with dermatopathology results: both suggested infection in 21 (14.0%) cases; both showed no evidence of infection in 85 (56.7%). The 150 sets of microbiology studies had a sensitivity of 80.8% (95% CI, 67.0%-89.9%), specificity of 59.2% (95% CI, 8.8%-68.9%), positive predictive value of 51.2% (95% CI, 40.0%-62.3%), and negative predictive value of 85.3% (95% CI, 74.2%-92.3%) for identifying infection.

Microbiology studies altered diagnosis in 72 of the 150 cases (48.0%); 68 cases showed no infection when infection was listed among the top differential diagnoses, and 4 cases showed infection when infection was not listed among the top differential. On return of microbiology results, antimicrobial drugs were started in 34 (22.7%) cases, including 27 where infection was listed among the top differential diagnoses but antimicrobial drugs were withheld until return of microbiological data, and in 2 cases where antimicrobial drugs were started based on clinical judgment despite negative microbiology results. Antimicrobial drugs were stopped in 1 (0.7%) case, altered in 4 (2.7%) cases, and triage/disposition recommendations were changed in 1 (0.7%) case.

These findings suggest that despite a low yield of true-positive results (42 of 150 cases; 28.0%), microbiological studies have high sensitivity and high negative predictive value and lead to changes in clinical management. Our results contrast with those of an earlier study, which found a low yield (1 of 158 true-positive)³ from microbiology studies conducted for immunocompromised cancer inpatients, most of whom were receiving antibiotics at the time of culture.^3,4

Our study is limited by its retrospective nature. In addition, our sensitivity and specificity calculations are based on sets of microbiological studies as opposed to individual studies. Despite this, our data suggest that routine tissue microbiology studies influence diagnosis and clinical management with reasonable sensitivity and negative predictive value. Future studies should evaluate which clinical features would enhance the yield and utility of microbiology studies and evaluate the cost-effectiveness of this practice.

Corresponding Author: Arash Mostaghimi, MD, MPA, MPH, Assistant Professor of Dermatology, Harvard Medical School, Brigham and Women’s Hospital, 75 Francis St, PBB-B 421, Boston, MA 02115 (amostaghimi@bwh.harvard.edu).

Published Online: August 30, 2017. doi:10.1001/jamadermatol.2017.3057

Accepted for Publication: June 23, 2017.

Author Contributions: Ms Xia and Dr Song are co–first authors. Ms Xia and Dr Mostaghimi had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Xia, Song, Mostaghimi.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Xia, Song, Mostaghimi.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Joyce.

Administrative, technical, or material support: Song, Mostaghimi.

Study supervision: Song, Mostaghimi.

Conflict of Interest Disclosures: None reported.