Required section · Section 2 of 6
What a Glucose or A1c Number Represents
Plasma glucose and A1c measure two different things about the same underlying condition. A plasma glucose result is a snapshot: the concentration in that specimen at the moment it was drawn. A1c is a weighted reflection of roughly the prior 2-3 months of glycation because red cells of different ages circulate together; recent weeks contribute more than earlier weeks. Neither is a direct measurement of the other, and a single elevated glucose does not require a matching elevated A1c on the same day.
The specimen matters as much as the analyte. Plasma glucose runs about 11% higher than whole-blood glucose at a normal hematocrit near 45%, because plasma carries a higher water content per volume than the red-cell compartment; the gap widens as hematocrit rises and narrows as it falls. A central-lab result is reported as plasma glucose. A point-of-care glucose meter intended for professional or home use is built by convention to already report a plasma-equivalent value, so its displayed number is meant to be compared directly against a laboratory plasma glucose, never multiplied again by a plasma-conversion factor.
From draw to result, a fasting glucose specimen passes through a defined sequence designed to protect the number from changing after it leaves the patient. That sequence is the model to follow: confirm the fasting state, collect into the correct tube, mix immediately, record the collection time, and separate the specimen from the cells promptly. Skip a step and the number on the screen may no longer represent the glucose concentration at the moment of the draw.
Before reading any glucose or A1c number as diagnostic, know what it is a snapshot of and what specimen handling stood between the draw and the report.
From a fasting glucose draw to a number the laboratory can stand behind.
Confirm the fasting state
Verify at least 8 hours with no caloric intake before the draw; water is allowed. A non-fasting draw cannot be read against the fasting threshold.
Collect into the glycolysis-inhibitor tube
Use a properly filled gray-closure, sodium fluoride-based tube for central-lab glucose. An underfilled or wrong-tube draw is a preexamination problem, not an analytical one.
Mix by gentle inversion immediately
Fluoride needs to reach the red cells to act. Delayed or absent mixing leaves glycolysis unopposed in part of the specimen.
Record the collection time
The interval between draw and separation is what the laboratory checks against its stability limits, so the clock has to be documented, not assumed.
Centrifuge and separate promptly
Sodium fluoride inhibits enolase, a late step in glycolysis, so glucose in a fluoride-only tube can still fall for roughly the first 1 to 4 hours before inhibition is fully effective. Prompt separation is what actually stops the drift.
Compare the result to the current criteria card
Only after the first five steps are satisfied does the plasma glucose value get compared against the fasting diagnostic threshold on the current-year criteria card.
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