The colorimetric readout in the described ALP assay is typically quantified by measuring absorbance on what basis?

• A. Absorbance at a specific wavelength
• B. Absorbance at multiple wavelengths
• C. Fluorescence intensity
• D. Radioactivity count

Answer: (A)

Colorimetric ALP assays rely on a chromogenic product that produces a color with a specific absorbance maximum. The enzyme converts the substrate into this colored product, so measuring absorbance at that single, characteristic wavelength (the color’s peak) directly reflects how much product has formed, and thus the enzyme activity. This approach follows Beer's law, linking higher absorbance to more product.

Other readouts aren’t typical for this type of assay: using multiple wavelengths is not necessary for the basic readout, fluorescence intensity applies to fluorescent-based assays rather than colorimetric ones, and radioactivity counts are for radiometric assays.