DMF Index
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Direct answer
Klein, Palmer and Knutson gave dental public health its workhorse in 1938: the DMF index counts permanent teeth that are Decayed, Missing or Filled due to caries, so DMFT of 3 means three teeth affected by the caries process in any of its treated or untreated forms. It is an irreversible, cumulative index — a tooth once counted stays counted — which is precisely why it suits surveys and why it cannot measure gum health or treatment quality. For primary teeth the lowercase def index is used, and the surface-counted DMFS version detects change more sensitively than the tooth-counted DMFT.
What you must remember
- Origin: Klein, Palmer and Knutson, 1938, for permanent teeth; Gruebbel proposed the def index for primary teeth in 1944, where e stands for indicated for extraction, since a missing primary tooth may simply have exfoliated.
- WHO codes for crown status on the oral health assessment form: 0 sound, 1 decayed, 2 filled with decay, 3 filled without decay, 4 missing due to caries, 5 missing for other reasons, 6 fissure sealant, 7 bridge abutment or special crown, 8 unerupted, 9 not recorded.
- DMFT counts teeth; DMFS counts surfaces (five surfaces per anterior tooth, eight per posterior tooth) and is the more sensitive instrument for detecting small changes.
- Interpretation scale for 12-year-olds: DMFT below 1.2 very low, 1.2–2.6 low, 2.7–4.4 moderate, 4.5–6.5 high, above 6.5 very high.
- The old WHO/FDI global goal was DMFT of no more than 3 at age 12; the Significant Caries index (Bratthall, 2000) highlights the one-third of a population with the highest DMFT, targeting hidden inequality behind a good mean.
- Strengths: simple, quick, universally comparable, valid for irreversible caries experience; limitations: assumes a missing tooth equals a one-surface filling in severity, ignores treatment need, cannot fall, and the M component becomes unreliable in older adults whose teeth were lost to periodontal disease.
- Root caries is measured separately, classically with Katz's Root Caries Index, in older age groups.
Scoring a mouth from code to conclusion
Score a 12-year-old boy tooth by tooth, which is how the viva and the survey both actually run. Tooth 16 has an occlusal cavity: code 1, one D. Tooth 26 has an intact amalgam on the occlusal surface: code 3, one F. Tooth 36 was extracted for pain last year: code 4, one M. Tooth 46 carries a fissure sealant: code 6 — not an F, a classic scoring trap. Tooth 38 is unerupted: code 8, excluded from any count. Everything else is sound. His DMFT totals 3, placing him in the moderate band for his age; the untreated component D is 1, so the care index F/(D+F) works out at one-half. Now translate: in a class of forty, if the mean DMFT is 2.1 but the SiC index is 5.8, the classroom mean hides a high-caries third who need targeted care — exactly the inequality argument the SiC index exists to make. Finally, if the same cohort is re-examined at 15 and the mean has risen to 2.9, that rise is genuine disease progression, because the index cannot decline with time.
Where students slip
The recurring mark-losers are coding decisions, not arithmetic. A filled tooth with recurrent caries at the margin is code 2 — it counts once, under D, never under both. A missing tooth in an adult over about 30 is not automatically caries-related; periodontal loss confounds the M component, which is why WHO scoring forces a judgement of reason for loss. A tooth missing because it was extracted for orthodontics or trauma is code 5 and scores nothing. On the primary side, an absent lower central incisor in a six-year-old is almost always exfoliation, not extraction, so it never enters def. And when asked "what does a DMFT of 2 tell you about treatment need?", the honest answer is nothing on its own — a filled tooth and a cavitated tooth both score 1 — a limitation examiners love because it separates reciters from thinkers.
Frequently asked questions
Who introduced the DMF index and when?
Klein, Palmer and Knutson in 1938 for permanent teeth; Gruebbel adapted it as the def index for primary teeth in 1944.
What is the difference between DMFT and DMFS?
DMFT counts affected teeth while DMFS counts affected surfaces, making DMFS more sensitive to small increments of new disease.
How does WHO code a fissure-sealed tooth?
Code 6 — a sealed tooth is neither decayed nor filled and contributes nothing to the DMF count.
What is the Significant Caries index?
Bratthall's 2000 measure: the mean DMFT of the one-third of the population with the highest DMFT, designed to expose inequality hidden by a favourable population mean.
Why does DMFT perform poorly in older adults?
Because teeth missing past middle age cannot reliably be attributed to caries, periodontal disease being a competing cause of tooth loss.