The Bleeding on Probing percentage (BOP %) is the most objective and reproducible measure of gingival inflammation. Introduced by Jukka Ainamo and Inkeri Bay in 1975, it records the proportion of sites that bleed when gently probed (≤0.2 N pressure) at the gingival margin.

Unlike subjective gingival indices (e.g., Loe-Silness GI), BOP % is dichotomous (bleeds / does not bleed), so inter-examiner reproducibility is high — a major reason it is preferred for longitudinal monitoring of periodontal therapy and for clinical trials.

• Use a calibrated WHO-621 / UNC-15 probe with light pressure (~0.2 N).
• Walk the probe gently along the sulcus at 4 sites (MB, B, DB, L) or 6 sites (MB, B, DB, ML, L, DL).
• Wait 10–15 seconds before scoring — delayed bleeding counts.
• Score each site as bleeding (+) or non-bleeding (−).
• BOP % = (bleeding sites / total sites probed) × 100

• < 10%: Periodontal stability (Lang et al 1990) — ideal target after therapy
• 10 – 25%: Localised gingivitis — reinforce technique, scaling
• 25 – 50%: Generalised gingivitis / early periodontitis
• ≥ 50%: Severe inflammation — high risk for attachment loss, referral consideration

Compared to GI (Loe-Silness) and PBI (Mühlemann), BOP % is faster and more reproducible. Compared to CPITN, BOP gives a continuous patient-level score rather than sextant categories — better suited for monitoring response to scaling and root planing. In India, IDA practice guidelines accept BOP % as the primary outcome for periodontal therapy.