Which reagent is used in a colorimetric method to quantify serum calcium?

• A. Cresolphthalein complexone
• B. Lanthanum
• C. Malachite green
• D. Amino-naphthol-sulfonic acid

Answer: (A)

Quantifying calcium by colorimetry relies on a dye that binds calcium to produce a colored complex whose intensity reflects the calcium concentration. Cresolphthalein complexone serves this role because, in alkaline conditions, it forms a colored complex with calcium (Ca2+), giving a deep purple color that can be measured spectrophotometrically. The strength and stability of this color provide a reliable, linear relationship between absorbance and calcium levels in serum, which is why this reagent is widely used in clinical assays.

Lanthanum, while used to reduce interference from phosphate in some calcium assays, is not the color-developing reagent itself. Malachite green is employed for detecting inorganic phosphate, not calcium, by forming a green complex. Amino-naphthol-sulfonic acid is not the standard reagent for calcium color development in routine serum calcium measurements.