Doctors use two tests for diagnosing nail infections. The KOH test, which involves looking at the nail-debris sample under a microscope, helps determine if the nail’s appearance is caused by a fungal or yeast infection or by another skin disorder. It’s a quick test. Results are usually available within 24 hours (and sometimes before you leave your doctor’s office).
The KOH test can’t determine exactly which type of dermatophyte fungi is causing the infection, however. For that, a fungal culture is needed. This test involves incubating the nail-debris sample in a laboratory and allowing it to grow. It’s then examined and tested to determine the specific microorganism involved. The process takes time. You may not receive the results for several weeks. Those results, though, will enable your doctor to choose the safest and most effective treatment course for you.
Should neither of these laboratory tests provide a clear diagnosis, your doctor may recommend additional tests, including, perhaps, a biopsy of the nail.