All toenail fungal infections are not created equal

Introduction to the Onychomycosis Severity Index (OSI)

Onychomycosis, a common disease of the nail unit caused by dermatophytes, nondermatophyte molds, and yeasts, has a prevalence of approximately 10-20% worldwide. Currently no standardized system exists to clinically grade the severity of onychomycotic nail disease. Such a scale is necessary for clinical trial inclusion criteria, for critical evaluation of treatment choice, and to help predict therapeutic outcome. Having quantitative numbers can also assist when discussing these factors with patients and other clinicians.

Inconsistences in onychomycotic grading systems can also lead to confusion about interpretation of efficacy. Recently, in a trial of ciclopirox olamine, mild to moderate disease was defined as 20% to 65% involvement of the nail plate. Most clinicians would argue that 20% area of involvement would be considered more moderate in severity, and also leaves questions on how to define disease involving less than 20% of the nail. In addition, area of involvement alone is only one factor to consider when assessing severity; a nail with limited involvement but significant thickness may have a poorer prognosis than a nail with more substantial, superficial involvement.

In 2011,a team of five dermatologists convened to develop an objective, reproducible, numeric grading system that describes the extent and involvement of distal subungual onychomycosis (DSO) and separates the nail involvement into defined mild, moderate, or severe categories. They created the Onychomycosis Severity Index (OSI). An OSI value is obtained by multiplying the score for the area of involvement (range, 1-5) by the score for the proximity of disease to the matrix (range, 1-5). Ten points are added for the presence of a longitudinal streaks or a patch (dermatophytoma), and another 10 points are added for presence of greater than 2 mm of subungual hyperkeratosis.

Mild onychomycosis corresponds to a score of 1 through 5 defines mild oncychomycosis, a score of 6 thorugh 15 is moderate, and a score between 16 to 35 defines severe onychomycosis.

Figure 1: Calculation of OSI






Figure 2: Proximity to matrix – scoring






Table 1: OSI Score and related classification

OSI Score Severity Classification
<=5 Mild
6 -15 Moderate
16 – 35 Severe

In conclusion, the OSI score is a newer, simple, objective and reproducible numeric system to grade the severity of onychomycosis. Practical application to foot care practitioners and clinics includes a standardized method of communication disease severity for referrals, clarity on disease severity when interpreting clinical trials, tracking patient progress during treatment, and also predicting response to treatment (thus managing patient expectations).  ToeFX adopted the OSI score in our principle clinical trial and continues to use this system in our ongoing trials. Please contact us if you have any questions on determining OSI score in your practice, we love hearing from our partner clinics and assisting in any way we can.


Carney C, Tosti, A, Daniel R, et al. A New Classification System for Grading the Severity of Onychomycosis: Onychomycosis Severity Index. JAMA Dermatology 2011; 147(11):1277-1282. doi: 10.1001/archdermatol.2011.267