Body Surface Area (BSA): Essential for dosing of dermatological therapeutics and quantitative assessment of dermatoses extent. Formula used: Mosteller.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Estimating the extent of cutaneous involvement for rashes, psoriasis, or burns.
Determining the feasibility of topical vs. systemic treatment.
Burn management triage (though Rule of Nines is faster, BSA estimation using the Palm method is more precise for scattered lesions).
Section 2
Formula & Logic
Methods of Calculation
Palm Method: The patient’s hand (including the fingers) represents approximately 1% of their total body surface area.
Palmar Surface: The palm alone (excluding fingers) is roughly 0.5% BSA.
Rule of Nines: Divides body into 9% blocks (e.g., each leg 18%, thorax 18%, abdomen 18%). Better for large contiguous areas like burns.
Section 3
Pearls/Pitfalls
Clinical Pearls
Always use the PATIENT'S hand size, not the examiner's hand size, to estimate 1%.
Overestimation is very common in psoriasis; spacing out plaques conceptually into a solid block helps visualize true BSA.
Section 4
Next Steps
Interpretation bounds (Psoriasis)
< 3% BSA: Mild disease. Generally managed with topicals.
3-10% BSA: Moderate disease. May require phototherapy or systemic oral agents.
> 10% BSA: Severe disease. Candidacy for biologic therapy is strongly considered.
