Blood glucose levels are usually tested in fasting samples of patients- Fasting Blood Sugar (FBS). The patient is required to fast for 8-12 hours before blood is extracted for testing. On certain occasions, Random Blood Sugar (RBS) or non-fasting samples are required too.
Oral Glucose Tolerance is also done in borderline suspected Diabetes Mellitus (DM) patients; sometimes an HbA1C test is done to monitor compliance to medication of patients.
Although automated procedures employing the enzymatic principle of glucose assays are now being adapted by many clinical laboratories, the manual method using the Orthotoluidine method is still useful. Some manual methods employ the oxidation-reduction property of glucose to measure its concentration.
In the Orthotoluidine method utilizes the principle of the condensation of aromatic amines by glucose to form a green end product which is then measured spectrophotometrically.
Unhemolyzed, non-icteric, non- tubid and non- lipemic serum is the best specimen. Serum is the supernatant fluid when coagulated blood is centrifuged. If an anticoagulant is used EDTA (ethylenediaminetetraacetic acid) is preferable.
The specimen and the orthotoluidine reagent is mixed and then boiled for 5-8 minutes, and then the absorbance or transmittance is read in the spectrophotometer. The reading will then be computed based on the absorbance /transmittance of unknown and standard and computed.
This formula is used to determine Cu (concentration of unknown):
Cu= Absorbance of Unknown/ Absorbance of Standard X Concentration of standard (Cu=Au/As X Cs)
This will give you the concentration in milligrams per deciliter. To convert this to mmol/L multiply it with 0.0555 (conversion factor)
The oral glucose tolerance test (OGTT) on the other hand, measures the body's ability to metabolize glucose. This test is used to diagnose borderline diabetes and also determines gestational diabetes mellitus (GDM) in pregnant women.
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