pharmcal: Microbial Assay Calculator

Microbial Assay Calculator (Antibiotic Potency) | PharmCal Professional

Microbial Assay Calculator

Calculate Antibiotic Potency via 2+2 Assay Method (USP <81>)

1. CONCENTRATIONS & ASSUMPTIONS

Std Low Dose (S1)

Std High Dose (S2)

Assumed Potency (%)

2. MEASURED RESPONSES (ZONE DIAMETERS)

Enter average zone size (mm) or absorbance for each level.

STANDARD (S)

Low Response (S1)

High Response (S2)

SAMPLE / TEST (U)

Low Response (U1)

High Response (U2)

ASSAY RESULTS

Calculated Potency: 0.00 %

Relative to Assumed: 0.00 %

Validity (Parallelism): Checking...

*Parallelism checks if slopes are similar (Diff must be minimal).

Performed By:

Name:

Date:

Checked By:

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Date:

Understanding the 2+2 Microbial Assay

Microbial assays (USP <81>) estimate the potency of antibiotics by comparing the growth inhibition of sensitive microorganisms caused by known concentrations of the antibiotic to be examined (Unknown) against a Reference Standard.

Step 1: Preparation

Prepare High and Low doses for both Standard ($S_2, S_1$) and Sample ($U_2, U_1$). The ratio of High/Low dose should be identical for both (usually 2:1 or 4:1).

Step 2: Measurement

Incubate and measure the Zone of Inhibition (mm) for Cylinder-Plate method or Absorbance for Turbidimetric method. Enter the average readings into the calculator.

Key Calculation Formula (Hewett Method)

M = ( (U1 + U2) - (S1 + S2) ) / Slope Factor × log(Ratio)

Potency = Antilog(M) × Assumed Potency

Validity Check: The calculator checks for "Parallelism". If the difference between the slopes of the Standard and Sample curves is significant, the assay may be invalid.

Labels: Microbial Assay Calculator