Limitations of a Rapid Antigen Detection Test in the Early Diagnosis of Group A Streptococcal Necrotizing Soft Tissue Infection

Abstract

Supplemental Digital Content is available in the text.

Sir,

Necrotizing soft tissue infection (NSTI) caused by group A streptococcus (GAS) is a life-threatening infection that shows rapid progression with acute vital corruption. Treatment requires an immediate diagnosis and debridement surgery. A rapid antigen detection test (RADT) for GAS has been used in pharyngitis with a high sensitivity and specificity.¹ In 1996, several reports showed the effectiveness of the RADT for NSTI wounds^2,3; however, care is required in evaluating the results of this test because its application for NSTI wound testing is a diversional use. We report a case of NSTI in which the RADT for GAS from blister fluid was negative, as an alert.

A 60-year-old woman was transferred to the emergency room with high fever. A physical examination revealed: temperature, 39.8°C; blood pressure, 134/95 mm Hg; pulse, 120 beats/min; respiratory rate, 40 breaths/min with 99% O₂ saturation on room air. Her left ankle was swollen and painful. A blood test showed the following values: WBC, 13,800/μl; PLT, 100,000/μl; CK, 447 IU/L; CRP, 18.9 mg/dl; HbA1c, 6.9%. Intra-blister fluid from her left ankle was sampled by tearing the membrane for both wound culture and RADT; RapidTesta StrepA (Sekisui Medical Co., Ltd., Japan) (Fig. 1). After the negative RADT, meropenem was administered intravenously under the suspicion of cellulitis. The next morning, her high fever continued with the spread of redness and pain to her left groin. We incised the dorsal foot skin to observe subcutaneous layer and took samples for a second RADT and wound culture (Fig. 2). A “finger test” was positive and “dishwater” pus was found. Moreover, the second RADT was positive, supporting the diagnosis of NSTI caused by GAS. Urgent debridement surgery was performed by removing skin and subcutaneous tissue from the dorsal foot and anterior lower leg and incising the medial side of the left knee and left thigh to exclude infection. (See figure, Supplemental Digital Content 1, which displays intraoperative findings on the second day (a). Postoperative findings at two months (b), http://links.lww.com/PRSGO/B472.) Penicillin G and clindamycin were administered intravenously for 7 days. The first wound culture from intra-blister fluid was negative; however, the second wound culture with a small incision identified GAS without other bacteria. Blood cultures on the first day were GAS-positive. Skin grafting surgeries were performed on the 14th and 49th day of hospitalization. Flexion and extension of all toes and the ankle were well preserved. She regained the ability to walk normally.

Fig. 1.

Findings of the first day. The white arrow indicates a blister on the left ankle. Intra-blister fluid was negative in both the RADT and wound culture.

Fig. 2.

Findings of the second day. The black arrow indicates the incised skin at the left dorsal foot. Subdermal fluid was positive in both the RADT and wound culture.

Immediate debridement surgery following an early diagnosis salvages the life and a limb; however, whether the intra-blister fluid should be used as a diagnostic sample has not been discussed. Intra-blister fluid samples have been used for RADT for GAS infection in both pediatric blistering distal dactylitis⁴ and NSTI.⁵ The possible reason for our negative result might be that the bacterial infection had not yet infiltrated the skin surface; however, the blood culture was already GAS-positive on the first day of hospitalization. We realized the limitation of applying intra-blister liquid samples in the early diagnosis and recommended obtaining a sample from the subcutaneous layer with a small skin incision.