Healthcare & Clinical Calculations
PASI (Psoriasis Area and Severity Index 0–72) & PASI-90 Response
Calculate Torsten Fredriksson and Ulf Pettersson’s 0.0–72.0 PASI score weighting Head (0.1), Upper Limbs (0.2), Trunk (0.3), and Lower Limbs (0.4) area grades (0–6) times plaque Erythema + Induration + Desquamation sums (0–12).
Compute exact 0–72 PASI scores and IL-17 / IL-23 biologic PASI-75 / PASI-90 clearance!
Introduced by Torsten Fredriksson and Ulf Pettersson (Dermatologica 1978), the Psoriasis Area and Severity Index (PASI, 0.0 to 72.0) is the universal regulatory endpoint for plaque psoriasis biologics (IL-17, IL-23, TNF, and TYK2 inhibitors)! Four anatomical body regions carry surface-area weights summing to 1.0: Head & Neck (0.10), Upper Extremities (0.20), Trunk including axillae/groin (0.30), and Lower Extremities including buttocks (0.40).
Within each region r, dermatologists grade Affected Area Grade A_r on a non-linear 0–6 scale (0 = 0%, 1 = 1–9%, 2 = 10–29%, 3 = 30–49%, 4 = 50–69%, 5 = 70–89%, 6 = 90–100%) and sum three 0–4 plaque severity signs (Erythema + Induration/Thickness + Desquamation/Scaling = S_r in 0–12): PASI = 0.1 * A_H * S_H + 0.2 * A_U * S_U + 0.3 * A_T * S_T + 0.4 * A_L * S_L! PASI >= 10 defines Moderate-to-Severe Plaque Psoriasis eligible for systemic/biologic therapy, while tracking percentage improvement from baseline PASI_0 measures PASI-75 (>= 75% reduction) and modern PASI-90 (>= 90% skin clearance)! Look at our severe baseline case (0.1*2*5 + 0.2*3*7 + 0.3*4*8 + 0.4*4*7 = 1.0 + 4.2 + 9.6 + 11.2 = 26.0): dropping to 1.7 after 16 weeks of IL-23 therapy achieves a 93.5% reduction (PASI-90 achieved)!
| Anatomical Region (Weight w_r) | Area Grade A_r (0–6) | Severity Sum E+I+D (0–12) | Weighted Regional PASI |
|---|---|---|---|
| Head (0.1) & Upper Limbs (0.2) | A_H = 2, A_U = 3 | S_H = 5, S_U = 7 | 1.0 + 4.2 = 5.2 |
| Trunk (0.3) & Lower Limbs (0.4) | A_T = 4, A_L = 4 | S_T = 8, S_L = 7 | 9.6 + 11.2 = 20.8 |
| Total Baseline PASI (0.0–72.0) | Sum of 4 Regions | >= 10 Biologic Threshold | 26.0 (Moderate-Severe Psoriasis) |
Total 4-Region PASI Score (0.0 to 72.0)
Validates regional Area grades [0, 6] and E+I+D severity sums [0, 12] before computing 0.1*H + 0.2*U + 0.3*T + 0.4*L.
=IF(OR(B2 < 0, B2 > 6, B3 < 0, B3 > 12, B4 < 0, B4 > 6, B5 < 0, B5 > 12, B6 < 0, B6 > 6, B7 < 0, B7 > 12, B8 < 0, B8 > 6, B9 < 0, B9 > 12), "Invalid PASI Grade", ROUND(0.1 * B2 * B3 + 0.2 * B4 * B5 + 0.3 * B6 * B7 + 0.4 * B8 * B9, 1))Percentage PASI Improvement From Baseline (%) for PASI-75 / PASI-90
Compares current PASI against pretreatment Baseline PASI in cell B10.
=IF(B10 <= 0, "Invalid Baseline PASI", ROUND(100 * (B10 - (0.1 * B2 * B3 + 0.2 * B4 * B5 + 0.3 * B6 * B7 + 0.4 * B8 * B9)) / B10, 1))Try the example
Use the original values or change them to test your understanding.