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Impact of systemic and topical antifungal therapy on the sensitivity of dermatophyte culture: A case–control study
Muhsin A Al-Dhalimi
1, Sarmad B Al-Dhalimi2
1Department of Dermatology, College of Medicine, University of Kufa, Iraq, 2College of Medicine, Jabir ibn Hayyan Medical University, Iraq
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ABSTRACT
Background: Dermatophytes are the most common cause of superficial fungal infections worldwide. Increasing evidence of antifungal resistance underscores the need for accurate fungal culture to guide species identification and susceptibility testing.
Objective: To evaluate the effect of ongoing or recently discontinued antifungal therapy on dermatophyte culture sensitivity.
Methods: A case-control study was conducted from October 2022 to October 2023. Patients with clinically suspected dermatophytosis and a positive potassium hydroxide (KOH) test were enrolled and categorized based on treatment status: no treatment (control), current topical corticosteroids, current antifungal therapy (topical and/or systemic), or discontinued antifungal therapy. Cultures were performed on Sabouraud Dextrose Agar and monitored for four weeks.
Results: Of 133 patients, 49 (36.8%) had positive cultures. The highest culture positivity was in the untreated control group (54.5%). Patients on systemic ± topical antifungals or who had discontinued them within one month showed significantly lower positivity rates (10.0% and 10.5%, respectively; p<0.05). Discontinuation beyond one month did not significantly affect results (33.3% positive). Topical antifungal use—whether ongoing, stopped <2 weeks, or stopped ≥2 weeks—did not significantly reduce culture sensitivity (33.3%, 50.0%, 57.1%, respectively). Topical corticosteroid monotherapy yielded a 28.6% positivity rate, not statistically different from controls.
Conclusion: Systemic antifungal therapy significantly reduces dermatophyte culture sensitivity for up to one month after discontinuation. Topical antifungals and corticosteroids do not significantly affect culture yield. These findings support withholding systemic antifungals for at least one month prior to culture, while topical agents may not require discontinuation.
Key words: Dermatophytosis, Fungal culture, Antifungal therapy, Culture sensitivity, Treatment effect
INTRODUCTION
Dermatophytes are the predominant cause of superficial fungal infections of the skin, hair, and nails worldwide [1]. These keratinophilic fungi are classified into three genera—Trichophyton, Epidermophyton, and Microsporum—based on morphological characteristics and natural habitat [2]. Dermatophytosis affects approximately 20–25% of the global population, with rising incidence and emerging antifungal resistance complicating clinical management [3–7].
Accurate diagnosis is essential for appropriate treatment. Direct microscopy with potassium hydroxide (KOH) offers rapid detection, but fungal culture remains the gold standard for species identification and antifungal susceptibility testing [8]. However, culture sensitivity can be influenced by prior or ongoing antifungal therapy [9]. Current guidelines recommend discontinuing topical antifungals for at least two weeks and systemic antifungals for four weeks before sampling, though evidence for these recommendations remains inconsistent [10,11]. Some authors advise stopping oral antifungal before doing fungal culture for as long as 3 months [12].
This study aimed to assess the impact of antifungal medications—both topical and systemic—on the sensitivity of dermatophyte culture, with the goal of informing evidence-based clinical sampling protocols.
PATIENTS AND METHODS
Study Design and Setting
A case-control study was conducted in the Dermatology Department of Al-Sadir Medical City, Al-Najaf, Iraq, from October 2022 to October 2023. Ethical approval was obtained from the Scientific Council of Dermatology and Venereology, Iraqi Board for Medical Specialties. Verbally informed consent was obtained from all participants or guardians.
Participants
Patients of any age with clinically suspected dermatophytosis and a positive KOH examination were included. Exclusion criteria were negative KOH, isolated nail involvement, or concurrent use of both antifungals and topical steroids.
Sample Collection and Processing
After cleansing the lesion with 70% alcohol, scales or epilated hairs were collected from the active border. A portion was used for KOH microscopy; the remainder was placed in a sterile container for culture.
Culture Methodology
Sabouraud Dextrose Agar (SDA) was prepared with cycloheximide (0.5 g/L) and chloramphenicol (0.05 g/L) to suppress saprophytic fungi and bacterial growth, respectively [13]. Inoculated plates were incubated at 27°C and examined twice weekly for four weeks. If no growth was obtained after four weeks, it was considered as negative culture. If any growth was obtained, photos were taken for the front and reverse surfaces of the colony for macroscopic classification. Dermatophyte identification was based on macroscopic colony morphology and microscopic features, confirmed by two microbiologists.
Treatment Groups
Patients were stratified into:
- No treatment (control)
- Topical corticosteroid monotherapy
- Current antifungal therapy (topical and/or systemic)
- Discontinued antifungal therapy
- Topical antifungals: stopped <2 weeks or ≥2 weeks
- Systemic ± topical antifungals: stopped <1 month or ≥1 month
Statistical Analysis
Data were analyzed using SPSS v27. Categorical variables were expressed as frequencies and percentages. Associations were tested using Pearson’s Chi-square or Fisher’s exact test. A p-value ≤0.05 was considered statistically significant.
RESULTS
Demographic and Clinical Characteristics
Among 133 patients with a clinical dermatophyte infection and positive KOH direct microscopic examination of the hair (Fig. 1), or skin (Fig. 2) were included in this study. The mean age was 29.0 ± 15.4 years, with the 30–39-year age group most frequently affected (25.6%). Males comprised 57.9% of the cohort. Tinea corporis (64.7%) and tinea cruris (40.6%) were the most common clinical presentations (Fig. 3). Combined body site involvement found 46 (34.6%) patients. The treatment status of the enrolled patient was categorised in Figure 4.
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Figure 1: Direct microscopic examination after KOH addition showed hyphae on the hair shaft at 4x. |
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Figure 2: Direct microscopic examination after KOH addition showed hyphae at 40x. |
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Figure 3: Distribution of the patients with fungal infection according to body site involvement. |
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Figure 4: The subdivisions of the patients according to the type of treatment. |
Culture Results by Treatment Group
Overall, 49 patients (36.8%) had positive cultures. The untreated control group had the highest positivity rate (54.5%). Patients on systemic ± topical antifungals or who had discontinued them within one month showed significantly reduced culture rates (10.0% and 10.5%, respectively; p<0.05). Discontinuation beyond one month did not significantly affect yield (33.3%).
Topical antifungal use—whether ongoing or recently stopped—did not significantly reduce culture sensitivity. Topical corticosteroid monotherapy yielded a 28.6% positivity rate, which was not statistically different from controls (p=0.103) (Table 1).
DISCUSSION
In the last few years, tinea has clinically presented in a wide range of manifestations closely mimicking a number of other skin diseases [14]. Additionally, there is increasing evidence of dermatophyte resistance to antifungal drugs [7]. These factors together increased the need for fungal culture for identifying the causative species and their drug susceptibility.
The findings of the present study confirm that systemic antifungal therapy significantly reduces dermatophyte culture sensitivity for up to one month after discontinuation. This aligns with recommendations to withhold systemic antifungals for at least four weeks before sampling [8]. In contrast, topical antifungals—whether ongoing or recently stopped—did not significantly affect culture yield, suggesting that current guidelines recommending a two-week washout period for topical agents may be overly conservative [10].
The lower but non-significant reduction in culture positivity with topical corticosteroids may reflect anti-inflammatory suppression of visible scaling rather than direct antifungal activity. These results support continued use of topical steroids when clinically indicated without compromising diagnostic culture.
The overall culture positivity rate of 36.8% despite positive KOH in all patients highlights the limitations of culture, likely due to non-viable hyphae or technical variations in sample handling [15]. T. rubrum and T. mentagrophytes predominance is consistent with global epidemiological trends [16–18].
Limitations
This study was conducted at a single center with a modest sample size. The use of verbal consent and convenience sampling may introduce selection bias. Additionally, antifungal drug levels were not measured, and the precise duration of effect may vary by agent.
CONCLUSION
Systemic antifungal therapy significantly impairs dermatophyte culture sensitivity for at least one month after discontinuation, supporting current washout recommendations. Topical antifungals and corticosteroids do not significantly affect culture yield and may not require discontinuation before sampling. These findings can guide clinicians in optimizing pretreatment protocols to improve diagnostic accuracy in dermatophytosis.
Statement of Human and Animal Rights
All the procedures followed were in accordance with the ethical standards of the committee responsible on human experimentation (institutional and national) and with the 2008 revision of the Declaration of Helsinki of 1975.
Statement of Informed Consent
Informed consent for participation in this study was obtained from all patients.
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