What is the essential difference between DMFT and DMFS?

Correct answer: B. DMFT scores each tooth as a single unit; DMFS scores each affected surface separately

• A. Third molar inclusion is a convention applied consistently to both indices, not a distinguishing feature.
• B. CORRECT. The T and S stand for TEETH and SURFACES. DMFT records a tooth as affected or not, giving a maximum of 28 (or 32 with third molars). DMFS records each affected SURFACE, giving a maximum of 128 — five surfaces on each posterior tooth and four on each anterior. DMFS is therefore considerably more SENSITIVE, since a tooth with one small lesion and a tooth with four separate restorations score identically on DMFT but very differently on DMFS.
• C. Both indices apply to the permanent dentition regardless of the patient's age.
• D. Neither index measures incidence or prevalence directly; both express cumulative caries experience.

Note: Use DMFS when you need to detect small differences — for instance in a clinical trial of a preventive agent, where DMFT would be too blunt to show an effect.