Glucose Estimation
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Direct answer
The grey-top tube exists because red cells are glucose thieves: without sodium fluoride to poison the glycolytic enzyme enolase, whole-blood glucose falls by roughly 5-7 mg/dL every hour on the bench. The reference estimation method is hexokinase (coupled to glucose-6-phosphate dehydrogenase, with NADPH formation read at 340 nanometres), while the commonest routine method is glucose oxidase-peroxidase — the GOD-POD or Trinder reaction — whose coloured end-product reads around 505 nanometres. Diagnostic thresholds common to WHO and ADA: fasting plasma glucose at or above 126 mg/dL, two-hour value at or above 200 mg/dL after a 75 g load, random glucose at or above 200 mg/dL with symptoms, or HbA1c at or above 6.5 per cent.
What you must remember
- Fluoride inhibits enolase and potassium oxalate anticoagulates; even so, glycolysis continues for roughly the first hour, so plasma should still be separated promptly.
- GOD-POD principle: glucose oxidase converts glucose to gluconic acid and hydrogen peroxide; peroxidase couples the peroxide to a chromogen, forming a coloured quinoneimine measured near 505 nm.
- Hexokinase is the reference method (specific, NADPH at 340 nm); Folin-Wu and Benedict's tests are history — Folin-Wu measured all reducing substances, Benedict's detects any reducing sugar, not just glucose.
- Classification: fasting plasma glucose below 100 normal; 100-125 impaired fasting glucose; at or above 126 diabetes (confirmed on two occasions, or with symptoms plus random glucose at or above 200).
- OGTT: 75 g anhydrous glucose in 250-300 mL water over five minutes after three days of adequate carbohydrate intake and an 8-14 hour fast; two-hour values under 140 normal, 140-199 impaired glucose tolerance, at or above 200 diabetes.
- Indian gestational diabetes convention (DIPSI, adopted in national guidelines): 75 g glucose given irrespective of fasting status; a two-hour value of 140 mg/dL or more diagnoses gestational diabetes mellitus.
- HbA1c at or above 6.5 per cent diagnoses diabetes and reflects 8-12 weeks of glycation; it is unreliable in haemolytic anaemia, recent transfusion, pregnancy and haemoglobinopathies such as thalassaemia.
- Point-of-care glucometers capillary readings guide management, but venous plasma in an accredited laboratory remains the diagnostic standard.
A glucose tolerance test, performed properly
A 26-week pregnant woman arrives for gestational diabetes screening under the DIPSI protocol: 75 g oral glucose is administered regardless of when she last ate, and a single two-hour venous sample is drawn into a fluoride tube. The value returns 152 mg/dL — at or above the 140 cut-off — and gestational diabetes mellitus is diagnosed; the pathway proceeds to medical nutrition therapy, glucose monitoring and, if targets fail, pharmacotherapy. The single-step design exists precisely for field-level feasibility across India's antenatal network.
Contrast a non-pregnant diagnostic OGTT. A man with a marginal fasting value of 112 mg/dL (impaired fasting glucose, since 100-125 spans that band) undergoes the full three-sample test: fasting 114, one-hour 198, two-hour 213 mg/dL — the two-hour value alone crosses 200 and establishes diabetes even though the fasting value never did. Every sample handling rule now matters: each tube is fluoride grey, spun and separated within the hour (a sample left on the shelf returns a falsely low two-hour value and can mask diabetes), and the timing clock runs from the last swallow of the drink, not from the phlebotomist's convenience. Interpretation then follows the numbers, not the impression of the glucose curve's shape.
Where students slip
Reporting "serum glucose" from a plain tube processed after lunch produces an artefactual hypoglycaemia; the fluoride tube plus timely separation is not optional. The band boundary trips many: 98 mg/dL fasting is normal and 100-125 is impaired fasting glucose — candidates who call 98 "prediabetic" have misplaced the threshold by a hair and a concept by a mile. Mixing whole-blood and plasma conventions is a third error: whole-blood glucose runs roughly 10-15 per cent lower than plasma, and modern meters report plasma-calibrated values, so laboratory and meter should agree, not differ mysteriously.
Frequently asked questions
Why is a fluoride tube used, and why still separate cells promptly?
Fluoride inhibits enolase and stops glycolysis only gradually — cells keep consuming glucose for about an hour, so prompt centrifugation still protects the true value.
State the GOD-POD principle with its wavelength.
Glucose oxidase generates hydrogen peroxide, which peroxidase links to a chromogen forming quinoneimine read around 505 nm — the Trinder reaction.
What are the diagnostic cut-offs for diabetes mellitus?
Fasting plasma glucose at or above 126 mg/dL, two-hour OGTT at or above 200, random glucose at or above 200 with symptoms, or HbA1c at or above 6.5 per cent.
What is the DIPSI criterion for gestational diabetes?
A 75 g glucose load given irrespective of fasting state; two-hour plasma glucose of 140 mg/dL or more diagnoses GDM in Indian national guidance.
Which method is the reference and which the routine?
Hexokinase (with NADPH measured at 340 nm) is the reference; GOD-POD is the routine endpoint method on most autoanalysers.
When is HbA1c unreliable?
Haemolytic anaemia, recent transfusion, pregnancy, and haemoglobinopathies such as HbE or thalassaemia common in India.