The amylase:creatinine clearance ratio (SCUA) is calculated as which formula?

• A. [ Serum Creatinine x Urine Amylase / Serum Amylase x Urine Creatinine ] x 100
• B. [ Serum Amylase x Serum Creatinine / Urine Amylase x Urine Creatinine ] x 100
• C. [ Urine Amylase x Urine Creatinine / Serum Amylase x Serum Creatinine ] x 100
• D. [ Serum Creatinine / Urine Creatinine ] x 100

Answer: (A)

Clearance is the volume of plasma from which a substance is completely removed per unit time. For amylase, you compare its clearance to that of creatinine to account for kidney function. The clearance of a substance X is Cx = (Ux × V) / Px, where Ux is urine concentration, V is urine flow, and Px is plasma concentration.

So amylase clearance divided by creatinine clearance is:

(C_A) / (C_Cr) = (U_A × V / P_A) / (U_Cr × V / P_Cr) = (U_A × P_Cr) / (P_A × U_Cr).

Expressing this as a percentage (multiply by 100) and substituting P_Cr as serum creatinine, P_A as serum amylase, U_A as urine amylase, and U_Cr as urine creatinine gives:

100 × (Serum Creatinine × Urine Amylase) / (Serum Amylase × Urine Creatinine).

This matches the correct formula.