Gram staining of infectious fluid shows high specificity for necrotizing soft tissue infection in a cohort study with prospective validation

A study with retrospective and prospective cohorts found that Gram staining of soft tissue infectious fluid had high specificity and positive predictive value for identifying necrotizing soft tissue infection (NSTI) in adults evaluated for suspected limb infection. The investigators described the results in the abstract of a study published in BMC Infectious Diseases on 21 September 2026.

The retrospective cohort included 22 patients with NSTI and 14 with severe non-necrotizing skin and soft tissue infection, while the prospective cohort included 19 patients with NSTI and 11 with non-necrotizing infection. Both cohorts comprised adult patients who presented to an emergency department with suspected limb NSTI and underwent ultrasound-guided aspiration of soft tissue infectious fluid for Gram staining and culture. Patients were classified according to their final discharge diagnoses. Ten of 76 patients who met the initial inclusion criteria were excluded after ultrasound-guided aspiration identified abscesses, leaving 36 patients in the retrospective cohort and 30 in the prospective cohort.

Bacterial presence on Gram-stained smears was more frequent among patients with NSTI in both cohorts, with reported p values of 0.011 in the retrospective cohort and 0.002 in the prospective cohort. Semi-quantitative Gram-stain scores were also significantly higher in patients with NSTI across the two cohorts. Culture positivity was higher in NSTI, with p < 0.001. In the abstract’s reported diagnostic analysis, the presence of bacteria on Gram-stained smears showed fair discrimination between NSTI and non-necrotizing infection, with area under the curve values ranging from 0.714 to 0.789. Specificity ranged from 92.9% to 100%, and positive predictive value ranged from 91.7% to 100%.

A semi-quantitative Gram-stain score of at least 2 had 100% specificity and 100% positive predictive value in both cohorts. The authors concluded that qualitative and semi-quantitative Gram staining may serve as rule-in diagnostic adjuncts for NSTI and could support clinical decision-making during initial evaluation. The available evidence comes from an abstract, so the full methods, diagnostic thresholds, confidence intervals and other study details could not be assessed here. The cohorts were also limited to adults with suspected limb NSTI who had fluid aspirated, and the reported performance does not establish that Gram staining alone definitively confirms or excludes the infection.

Subject of Research: Diagnostic performance of Gram-stained soft tissue infectious fluid for identifying necrotizing soft tissue infection in adults with suspected limb infection

Article Title: Diagnostic performance of gram-stained soft tissue infectious fluid in necrotizing soft tissue infection: a cohort study with prospective validation

Article References: Wu, K.-H., Chang, C.-P., Lin, L.-C., Wu, P.-H., Hsiao, C.-T., Hsu, Y.-S., Lin, Y.-T., Hung, C.-H., Lee, C.-Y., & Chang, P.-J. (2026). Diagnostic performance of gram-stained soft tissue infectious fluid in necrotizing soft tissue infection: a cohort study with prospective validation. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14499-6

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14499-6

Keywords: necrotizing soft tissue infection, Gram staining, infectious fluid, diagnostic performance, prospective validation, retrospective cohort, skin and soft tissue infection, sensitivity analysis, positive predictive value, ultrasound-guided aspiration, Diagnostic, performance