Fasting Glucose in a Delayed Plain Tube
A 46-year-old woman with a body-mass index of 31 kg/m² and a mother with type 2 diabetes attends a clinic for a fasting blood test. Blood is drawn at 07:30 into a plain serum tube, not a fluoride tube. The clinic is busy; the sample stays on the reception desk at room temperature and reaches the laboratory at 11:45. The laboratory separates the serum and measures glucose by the hexokinase method: 5.4 mmol/L (97 mg/dL). The clinician had expected an impaired fasting glucose result and asks whether the value is reliable. The laboratory’s information sheet states that glucose in whole blood falls by about 5–7% per hour at room temperature unless glycolysis is inhibited. The patient has no symptoms of hypoglycaemia and no history of diabetes. A repeat fasting sample is possible next week. No HbA1c has been requested.
1. Is the reported glucose reliable? Estimate the likely original glucose and explain the direction of error.
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No. In a plain tube, red cells continue glycolysis. At 5.4 mmol/L, 5–7% per hour is roughly 0.27–0.38 mmol/L per hour. Over 4.25 hours from 07:30 to 11:45, the fall is about 1.1–1.6 mmol/L, so the original fasting glucose was probably around 6.5–7.0 mmol/L, in the impaired fasting glucose range of 5.6–6.9 mmol/L. The reported 5.4 mmol/L is falsely low and must not be used to rule out dysglycaemia. The estimate is approximate, so it cannot be used to diagnose diabetes or IFG.
2. What specimen and handling should be used for the repeat, and why?
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Use a sodium fluoride/oxalate tube or a citrate tube; fluoride inhibits enolase and blocks glycolysis. If a plain tube is unavoidable, centrifuge within 30–60 minutes and separate serum promptly, then refrigerate. For plasma glucose, separate within 30 minutes. The repeat should be a true fasting sample of at least 8 hours. Document collection and receipt times. HbA1c can be added, but it reflects 2–3 months and is not a substitute for proper sample handling.
3. What result would confirm IFG or diabetes, and what would you do next?
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On a properly handled fasting plasma glucose: 5.6–6.9 mmol/L indicates IFG; 7.0 mmol/L or more indicates diabetes and requires confirmation on a second sample unless there is unequivocal hyperglycaemia. A 2-hour OGTT glucose of 7.8–11.0 mmol/L is impaired glucose tolerance; 11.1 mmol/L or more is diabetes. HbA1c 5.7–6.4% is prediabetes; 6.5% or more is diabetes. Because the current sample is invalid, repeat FPG with fluoride and add HbA1c; do not treat or reassure on the 5.4 mmol/L.