Mycological tests and onychomycosis

Abstract

SYNOPSISThe value of direct microscopy and cultural tests in onychomycosis is reported on the results of the examination of 3,955 nail collections from patients infected with Trichophyton rubrum. Of the total number of mycological diagnoses, 15 % would have been missed if culture had been omitted and 53 % if direct microscopy had not been undertaken. METHODSIn the mycological diagnosis of onychomycosis tissue may be examined either by culture or by the so-called direct method, in which nail parings, after hydrolysis in potassium hydroxide solution, are squashed to a more or less unicellular layer and then searched microscopically for the presence of hyphae or mycelium. In some laboratories direct examination is performed only when no fungi have been grown within a period varying from three to six weeks from the planting of the nail in agar media; in others culture is undertaken only when direct examination has shown fungus to be present, and there are yet other laboratories performing both tests for every specimen.In our routine for the investigation of nail parings, three or four pieces are examined in 30% KOH, three are planted in peptone glucose agar containing 40 utg/ml chloramphenicol, and six in malt extract agar containing 40 zg/ml chloramphenicol and 0 5 mg/ml of cycloheximide. All cultures are incubated at 25'C for six weeks before any are discarded as negative. No direct preparation is recorded as negative until it has been searched for a minimum of ten minutes unless the amount of material is inadequate.This paper reports an analysis of the results of examining 3,955 samples of nails from patients with Trichophyton rubrum infections during a therapeutic trial of griseofulvin (Davies, Everall, and Hamilton, 1967). Table I shows the relative numbers of mycological diagnoses by direct microscopy, culture, or both. RESULTSOnly 913 (46%) of the nails judged to be infected by such clinical criteria as hyperkeratosis and discoloration showed fungus by microscopy or culture or both (Table I). Had greater care been taken in collection and a greater amount tested from each Received for publication 21 March 1968. nail, the figure might have been higher, but fungi are not the sole cause of nail dystrophy. On the other hand it is noteworthy that 170 (9 %) of the normal, healthy looking nails from infected patients showed fungus by one or other method. Of 2,001 nails regarded as infected by the clinician, 803 (40%) showed fungus by direct microscopy but in only 313 (39%) of these was T. rubrum isolated. The difficulty in isolating fungus from nails is well recognized (Walshe and English, 1966)

Forty-Eight-Hour Diagnosis of Onychomycosis with Subtyping ofTrichophyton rubrumStrains

“…As a "gold standard," KOH and culture is problematic: various studies have shown that 15 to 50% of the microscopy-positive nail specimens do not grow a culture (1,6,8,33). In our nail study (Table 2), the failure of culture was even more severe: a full 72.1% of PCR-confirmed dermatophytosis specimens (i.e., 109 of 151 specimens) did not yield the dermatophyte in culture.…”

“…These methods are low in specificity and may be very timeconsuming, because growth, sporulation, and routine physiological testing (e.g., urease enzyme assay) of the fungi involved may take 2 to 4 weeks. In addition, 15 to 50% of microscopically true-positive samples fail to grow a culture (8,33). Moreover, dermatophyte species show an unusual level of variation in the expression of phenotypic characteristics, e.g., the formation of pigments or conidia (46).…”

Utility of Inoculum Counting (Walshe and English Criteria) in Clinical Diagnosis of Onychomycosis Caused by Nondermatophytic Filamentous Fungi

“…It is always possible with mold infection, as with dermatophytosis, to sample fortuitously a portion of the lesion that is suboptimal for laboratory investigation, whether the area sampled contains effete inoculum, or whether it is marginal and is not yet heavily colonized. The laboratory study of dermatophytosis is constantly rendered difficult by the failure of truepositive samples to contain living inoculum in 15 to 25% of cases (4,6,7,13,32,34). In light of such possibilities, it may be unrealistic to expect low mold inoculum counts to be as readily interpretable as high counts are.…”

Methods for Diagnosing Onychomycosis

“…Thus, staining methods have a higher yield than culture techniques in these patients. 23 Our results showed that PATHPAS had the highest frequency of positive results (40 patients [63%]) in this sample population. This is lower than one would expect based on the clinical appearance of the nails in our patients.…”

“…[25][26][27] The literature regarding evaluation of diagnostic tests for onychomycosis dates back to 1968. Davies 23 examined the frequency of positive results of KOH examination and culture for Trichophyton rubrum in 3955 patients for whom cultures had previously been positive for T rubrum onychomycosis. In that study, 46% of specimens showed positive culture or KOH examination.…”