Clinical Benefit of Recovering Dermatophytes from Skin Swabs Sent for Bacterial Culture

Abstract

We incubated Sabouraud dextrose agar plates to recover dermatophytes from skin swabs sent for bacterial culture. Dermatophytes were recovered from 66 (0.3%) of 22,613 cultures. Twenty-one patients received specific antifungal treatment when their dermatophyte was reported. Most clinicians thought recovering and reporting the dermatophyte contributed to patient management.

Diagnostic Medlab receives specimens from community health care providers, mostly general practitioners, in Auckland, New Zealand. Six percent of our specimens are swabs from superficial skin sites. Because of the range of lesions encountered in community practice, in addition to culturing for bacteria we inoculate swabs onto a quadrant of Sabouraud dextrose agar (SDA) containing chloramphenicol (100 mg/liter) and gentamicin (40 mg/liter) and incubate in air at 35°C for 48 h to recover yeasts of clinical relevance. This report summarizes the clinical benefit gained by incubating the SDA plate longer to recover dermatophytes that would otherwise be missed.

Plates are read at 24 and 48 h, and the presence of bacterial or yeast pathogens is reported. For 5 months the SDA plate was incubated a further 7 days aerobically at 28°C. Plates were examined, and dermatophyte isolates were identified by macro- and micromorphology (3) and reported.

The clinical relevance of the dermatophyte was determined 3 to 4 weeks after being reported by sending a fax to the patient's doctor seeking information on empirical treatment given when the swab was taken, treatment given following the bacterial or dermatophyte culture result, clinical response to any treatment(s) used, and whether the dermatophyte result was helpful in patient management and seeking any other comment. If there was no response to the fax, one of us phoned for the information.

The SDA plates from 22,613 swabs of superficial sites were inspected for the growth of a dermatophyte. It took approximately 3 min a day to inspect the plates, four inocula per plate, and approximately 45 min a week to identify the isolates. The following dermatophytes were recovered from 66 (0.3%) patients: Trichophyton rubrum, 42 patients (64%); Trichophyton mentagrophytes, 16 patients (24%); Microsporum canis, 4 patients (6%); Epidermophyton floccosum, 4 patients (6%). The infected sites were as follows: feet or toes, 29 patients (44%); groin or buttock, 12 patients (18%); leg, 10 patients (15%); trunk, 6 patients (9%); arms or hands, 5 patients (8%); face, 4 patients (6%). The median patient age was 44 years (range, 4 to 96 years); 34 (52%) were male.

Table 1 summarizes the empirical treatment, bacterial culture result, and patient management response following the report of the dermatophyte isolate for the 65 patients with follow-up information. Twenty-one (32%) of the 65 patients received antifungal treatment as the result of the dermatophyte isolate being reported to their clinician. Twenty (56%) of the 36 patients who had not received empirical antifungal treatment were prescribed an antifungal in response to the isolation of their dermatophyte.

TABLE 1.

Empirical treatment, bacterial culture result, and management response to dermatophyte culture result for 65 patients with a dermatophyte recovered after an additional 7-day incubation of SDA plates

Empirical treatment	n	No. of patients:
		With bacterial culture result of:		Receiving treatment for the dermatophyte isolate after the dermatophyte culture result was received
		No pathogen	Pathogen
No treatment	12	9	3	9
Antibacterial only	19	11	8	9
Antifungal only	15	8	7	0
Combination antibacterial and antifungal	14	6	8	1c
Othera	5	2	3	2
Total	65	36	29b	21

^aSteroid (n = 4) or emollient cream (n = 1).

^bS. aureus alone (n = 19 patients), S. aureus with Streptococcus pyogenes (n = 4) or Pseudomonas aeruginosa (n = 3), Proteus sp. (n = 1), or β-hemolytic streptococci, group B (n = 1) or G (n = 1).

^cA patient with a third relapse following topical antifungal agents was treated with an oral antifungal agent.

Sixteen patients (3 receiving no empirical treatment, 10 receiving empirical antibacterial treatment, and 3 receiving other empirical treatment) did not receive treatment for the dermatophyte: 10 had responded to their empirical treatment, 2 were not contactable, 1 had died, 1 had spontaneous resolution of the lesion, 1 had a trivial lesion not warranting treatment, and 1 had received treatment elsewhere.

Most clinicians, 45 (69%), responded “yes” to the question “Did the report of the presence of the dermatophyte help you in managing the patient?” For patients who had not responded to empirical antibacterial treatment the report of a dermatophyte prompted antifungal treatment; e.g., a pustular forehead lesion thought to be a Staphylococcus aureus infection was shown to be caused by M. canis. Most clinicians, 22 of 29 (76%), who had prescribed empirical antifungal treatment reported that the result was helpful in patient management.

Depending on the body site and visual appearance, the differential diagnosis of a skin lesion may include a noninfectious condition, a secondarily infected noninfectious condition, erythrasma, candidiasis, dermatophytosis, or bacterial infection. The appearance of the lesion can be modified by medications of which the clinician may be unaware. For these reasons we include SDA among the primary media used for superficial body swabs sent primarily to detect bacterial or candida infection.

This study was initiated to see if clinically relevant dermatophytes could be detected by incubating SDA plates a week longer than the usual 48 h to recover yeasts.

We incubate fungal cultures, from specimens sent specifically for mycology, for 3 weeks. For the year 2000 we reported 3,537 dermatophytes; 83, 16, and 1% were reported after incubation for 1, 2, or 3 weeks, respectively. These data add to previous studies showing that most fungal isolates are recovered within the first 2 weeks of incubation (1, 2). We therefore chose to hold the SDA plates an additional 7 days, giving a total of 9 days of incubation. This decision took into consideration incubator space and the need for timeliness of the report for clinicians.

Over one-half of the patients from whom a dermatophyte was isolated and who had not received empirical antifungal treatment had their dermatophyte isolate treated. Most clinicians, including those who had given empirical antifungal treatment, thought that the report provided information helpful for patient management. Although most of the clinicians who did not find the report helpful had patients who responded to empirical treatment, several of them stated that the result would have been useful had the patient not already responded. The value of reporting dermatophytes is best represented by those patients who received either no empirical treatment or antibacterial treatment, where less than one-half had a bacterial pathogen isolated from the skin swab (Table 1).

We emphasize that our approach to recover dermatophytes from skin swabs sent for bacterial culture is an attempt to maximize clinically relevant culture information from what we acknowledge is an inferior specimen type for fungal recovery. Our results must not be taken as suggesting that this approach, to make the most of the specimen received, can replace proper mycology methods when fungal infection is suspected. If a fungal etiology is being specifically sought, adequate specimen collection, direct examination, and appropriate culture and incubation methods must be used (4).

On the basis of clinician feedback we continue to report dermatophytes recovered after additional incubation of SDA plates. For the 12-month period October 2002 through September 2003, 224 dermatophytes (0.4%) were reported from 56,195 skin swabs. Our findings may be relevant to other laboratories serving community-based clinicians, who are often faced with skin lesions of uncertain etiology or atypical appearance.