Glucose estimation (GOD-POD method)

Principle

• Glucose oxidase (GOD) oxidises β-D-glucose to gluconic acid and hydrogen peroxide (H2O2).
• Peroxidase (POD) uses the H2O2 to couple 4-aminoantipyrine with phenol, forming a red-violet quinoneimine dye (Trinder reaction).
• Colour intensity, read at about 505 nm (500–510 nm), is proportional to glucose concentration.
• GOD is specific for β-D-glucose; mutarotase may be added to speed conversion of the α-anomer.

Specimen

• Fluoride-oxalate (grey top) plasma is preferred; fluoride inhibits enolase and stops glycolysis.
• In unseparated blood without fluoride, glucose falls roughly 5–7% per hour at room temperature.
• Serum or heparin plasma is acceptable if separated promptly; CSF and other fluids can also be tested.

Procedure

• Typical: 10 µL sample or standard + 1.0 mL working reagent; mix.
• Incubate at 37 °C for about 10 minutes (or per kit at room temperature).
• Read test and standard against a reagent blank at 505 nm.
• Glucose = (A test / A standard) × standard concentration (commonly 100 mg/dL).

Results

• Typical fasting plasma glucose: 70–99 mg/dL (3.9–5.5 mmol/L); local reference ranges apply.
• Fasting 100–125 mg/dL (5.6–6.9 mmol/L): impaired fasting glucose (ADA).
• Fasting ≥126 mg/dL (7.0 mmol/L), or random ≥200 mg/dL (11.1 mmol/L) with symptoms: diabetes.
• Conversion: mg/dL ÷ 18 = mmol/L.

Quality control and pitfalls

• Reducing substances (ascorbic acid, uric acid, bilirubin) compete for H2O2 and give falsely low results.
• Haemolysis and lipaemia interfere; results above linearity need dilution.
• Run normal and abnormal controls with each batch.

Clinical use

• Diagnosis and monitoring of diabetes mellitus, hypoglycaemia and CSF glucose in meningitis.