Principle of Measurement

1. Arsenazo III Method (Most widely used in modern analyzers)

How it works:

• Calcium ions react with Arsenazo III dye to form a blue-purple complex.

• The intensity of the color is measured photometrically at 650–660 nm.

• Highly specific for calcium with minimal interference from magnesium.

Advantages:

• Excellent stability

• Strong absorbance

• High specificity and precision

2. o-Cresolphthalein Complexone (OCPC) Method

How it works:

• Calcium forms a purple-red complex with o-cresolphthalein complexone in alkaline conditions.

• Read photometrically at 570–580 nm.

• Common in semi-auto chemistry analyzers.

Advantages:

• Reliable and widely established

• Suitable for manual and automated systems

Reagent Components

Arsenazo III Reagents May Contain:

• Arsenazo III dye

• Buffer (MES or other suitable buffers)

• Stabilizers

• Surfactants

• Preservatives

OCPC Reagents May Contain:

• o-Cresolphthalein complexone

• Alkaline buffer (e.g., 8-hydroxyquinoline to remove magnesium interference)

• Dye stabilizer

• Surfactants

• Preservatives

Packaging Format

• Single liquid reagent (most common)

• Two-reagent system (R1 + R2)

• Ready-to-use liquid stable bottles

• Analyzer-specific reagent cartridges

Common volumes: 25 ml, 50 ml, 100 ml, 250 ml or more.

Sample Types

• Serum

• Plasma (heparinized samples preferred)

• Urine (dilution may be needed)