From the Guidelines
To diagnose nail fungus, a fungal culture test or direct microscopic examination with Calcofluor white is typically used, as it provides the most accurate results. The diagnosis of nail fungus, also known as onychomycosis, requires laboratory testing to confirm the presence of a fungal infection 1. The clinical characteristics of dystrophic nails must alert the clinician to the possibility of onychomycosis, and laboratory detection and identification of dermatophytes consists of culture and microscopy 1.
Some key points to consider when diagnosing nail fungus include:
- The use of Calcofluor white for direct microscopic examination of nail specimens, as it is exceedingly useful for seeing fungal elements 1
- The importance of taking a small sample of the affected nail or debris beneath it and sending it to a laboratory to see if fungus grows 1
- The need for an experienced microscopist and a laboratory with expertise in mycology to accurately interpret the results 1
- The potential for incorrect diagnosis if laboratory testing is not used, as clinical diagnosis alone can be unreliable 1
In terms of specific tests, the following may be used:
- Fungal culture test, which involves taking a small sample of the affected nail or debris beneath it and sending it to a laboratory to see if fungus grows 1
- Potassium hydroxide (KOH) preparation, where nail scrapings are placed in a solution and examined under a microscope to identify fungal elements 1
- Direct microscopic examination with Calcofluor white, which provides the most accurate results 1
From the FDA Drug Label
Prior to initiating treatment, appropriate nail specimens for laboratory testing (KOH preparation, fungal culture, or nail biopsy) should be obtained to confirm the diagnosis of onychomycosis. The tests used to see if there is nail fungus are:
From the Research
Diagnostic Tests for Nail Fungus
The following tests are used to diagnose nail fungus:
- Direct microscopic examination with a potassium hydroxide (KOH) wet-mount preparation 3, 4, 5
- Histopathologic examination of the trimmed affected nail plate with a periodic-acid-Schiff (PAS) stain 6, 3, 4, 5
- Fungal culture 6, 7, 3, 4, 5
- Polymerase chain reaction (PCR) assays 5
Sensitivity and Specificity of Diagnostic Tests
The sensitivity and specificity of these tests vary:
- PAS staining is highly sensitive (93.1%) 3
- KOH examination is highly sensitive but poorly specific 4
- Fungal culture is highly specific but poorly sensitive 3, 4
- Gomori's methenamine silver (GMS) stain is both highly sensitive and specific 4
Diagnostic Algorithm
A diagnostic algorithm for onychomycosis has been proposed, taking into consideration the sensitivity, cost-effectiveness, and necessary time for each test 3. The algorithm suggests using PAS staining as a primary diagnostic test, followed by fungal culture or KOH examination as secondary tests.