Differential diagnosis. Candidiasis, intertrigo, and erythrasma can cause similar lesions. It is helpful to recall that candidiasis may involve the scrotum and penis while tinea cruris does so rarely. An additional helpful feature is the characteristic bright coral appearance of erythrasma when examined with a Wood’s lamp, absent in cases of tinea cruris.
Tinea corporis and tinea faciale
Tinea infections of the face and body begin as flat, scaly, and often pruritic macules that subsequently develop a raised border and begin to spread radially. As the ring expands, the central portion of the lesion often clears. This pattern leads to the formation of irregular circles that gives tinea corporis its common name, ringworm (Figure 2).
Tinea faciale is less common, but generally has a similar appearance with central clearing of lesions. Tinea faciale may not always exhibit the sharply demarcated border of tinea corporis.
Differential diagnosis. Eczema, impetigo, early pityriasis rosea, and localized psoriasis can mimic tinea corporis. History of exposure to persons or animals (typically house pets) with known ringworm should increase suspicion of tinea corporis. Current or past evidence of psoriasis or eczema should broaden the differential to include atypical presentations of these diseases.
FIGURE 2
Tinea corporis
Widespread tinea corporis. This patient would not be a candidate for topical treatment.
Diagnosing tinea: history and exam not enough
In some cases, findings on history and physical evaluation are so characteristic of tinea that they alone may allow the clinician to make a firm diagnosis. However, studies have shown that relying upon history and physical examination may result in a many missed diagnoses.6 Accordingly, consider tinea in all instances of papulosquamous skin disease and follow up appropriately. Figure 3 presents a simple algorithm for diagnosis and treatment of suspected tinea infection.
FIGURE 3
Evaluating possible tinea infection of the skin
Koh preparation
An important adjunct in diagnosing tinea is the office-based KOH wet mount preparation ( Figure 4 ), permitting direct visualization of fungal hyphae in keratinized material of the stratum corneum. Detailed protocols for preparation and interpretation of the KOH slide for diagnosis of dermatophytosis are well described in many other sources.10
FIGURE 4
KOH preparation
Potassium hydroxide dissolves epithelial cells to reveal hyphae (A), with their characteristic branching pattern (B). To increase the likelihood of detecting hyphae on microscopic exam of skin scrapings, begin with low light and low power. If fungal elements are detected, increase magnification to confirm the diagnosis. If fungal elements are not detected, yet clinical suspicion of tinea is high, consider arranging for a fungal culture.
Performing the test
The slide is prepared by combining skin scrapings from the leading edge of a lesion with a small amount of 20% potassium hydroxide. A cover slip is applied and the slide is gently heated prior to microscopy examination under high power. Table 2 summarizes the utility of clinical diagnosis and KOH preparation in diagnosis of tinea infection. History and physical examination should be combined with KOH preparation when making the diagnosis of epidermal dermatophyte infection (strength of recommendation [SOR]=B).6 (For an explanation of evidence ratings, see page 865.)
TABLE 2
Diagnostic value of clinical diagnosis and KOH prep in tinea infection
| Test | Quality | Sensitivity | Specificity | PV+ | PV– | LR+ | LR– |
|---|---|---|---|---|---|---|---|
| a. Clinical diagnosis6 | 2b | 81% | 45% | 24 | 92% | 1.47 | 0.42 |
| b. KOH prep50 | 2b | 88% | 95% | 73% | 98% | 17.6 | 0.13 |
| c. KOH prep51 | 2b | 77% | 62% | 59% | 79% | 2.02 | 0.37 |
| a. The clinical diagnosis set was compiled by 27 general practitioners prior to submission of skin for fungal culture. Specimens were taken from consecutive patients with erythrosquamous lesions. Culture results were considered the gold standard. Study quality=2b. | |||||||
| b/c. Both studies of KOH preps were open analyses of patients with suspicious lesions. Paired fungal culture was initiated simultaneously with KOH prep and was considered the gold standard. Study quality=2b. For an explanation of levels of evidence, see page 865. | |||||||
Caveats
Though a rapid and inexpensive test, the KOH preparation has limitations. Some physicians have little experience interpreting KOH preparations, thus reducing the value of the test. Some patients with clinically significant tinea infections may be using over-the-counter topical antifungal medications, thus reducing the likelihood that fungal hyphae will be visualized at the office visit.
In cases where clinical suspicion of tinea infection is high, and the result of a KOH prep is negative, tissue should be submitted for fungal culture. Fungal culture is not typically available in the primary care office setting and usually requires submission of the sample to a reference or hospital laboratory.
Dermatophyte test medium
Dermatophyte test medium may be prepared and used in the office. It is mixed with a phenol indicator that turns red on exposure to alkaline metabolites of fungal growth. This medium is sensitive and usually exhibits color change within one week.11 Most primary care offices do not use dermatophyte test medium, however, because of its short shelf life and because the Clinical Laboratory Improvement Amendments classification requires that this test be performed by qualified laboratory personnel.