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Glucose and the diabetes criteria

16 min

Skills you'll build3 skills
  • Classify a glucose or HbA1c result using the ADA limits and confirmation rule
  • Match each glucose pathway to energy release, glucose storage, or new glucose synthesis
  • Choose prompt separation, an ice slurry, or an inhibitor tube to stop glucose loss

At the bench: Assess specimen interference before reporting a chemistry result

Try first

Try first

An adult with no symptoms has one fasting plasma glucose of 131 mg/dL from a properly handled specimen. No other glucose or HbA1c result exists. How does this result classify under the American Diabetes Association (ADA) criteria?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Glucose metabolism

Four pathways move glucose between use, storage and new supply:1

  • Glycolysis breaks glucose down to pyruvate for energy. Red cells depend on it.
  • Glycogenesis stores glucose as glycogen in liver and muscle.
  • Glycogenolysis breaks glycogen down. The liver has glucose-6-phosphatase and releases the glucose into blood. Muscle uses its glycogen for its own energy needs.
  • Gluconeogenesis builds new glucose in the liver and kidney from lactate, glycerol and amino acids.

Insulin favors uptake and storage. Glucagon drives glycogen breakdown and gluconeogenesis. Early in a fast, liver glycogen maintains plasma glucose. As the fast goes on and glycogen runs low, gluconeogenesis supplies more of it.1

Glycolysis in the specimen tube

Blood cells continue to consume glucose after collection, and a high cell count speeds the loss. Three measures limit it:2

  1. Separate the plasma from the cells promptly.
  2. Put the tube in an ice-water slurry and separate the plasma within 30 minutes.
  3. Collect into a validated citrate-fluoride-EDTA tube. The citrate acidifies the sample and stops glycolysis promptly.

Sodium fluoride does not promptly prevent glucose loss, so clinically important glucose loss can occur during the first hours after collection.2 Treat a delayed low glucose as a handling problem until the collection and separation times are confirmed.

The ADA limits and the confirmation rule

The ADA classifies laboratory plasma glucose and hemoglobin A1c (HbA1c) against fixed decision limits. A fasting glucose needs at least 8 hours without caloric intake.3

TestPrediabetesDiabetes
HbA1c5.7–6.4%6.5% or higher
Fasting plasma glucose100–125 mg/dL126 mg/dL or higher
2-hour glucose, 75-g oral glucose tolerance test140–199 mg/dL200 mg/dL or higher
Random plasma glucoseNot used200 mg/dL or higher with classic symptoms or a hyperglycemic crisis

Without unequivocal hyperglycemia, diabetes needs two abnormal results. The second can come from a prompt repeat of the same test or from a different test.3 A random glucose of 200 mg/dL or higher with classic symptoms or a hyperglycemic crisis is sufficient without a second result.3

Follow the bench demonstration

Sources for this step
References
  1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
  2. Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Diabetes Care. 2023;46(10):e151-e199. doi:10.2337/dci23-0036
  3. American Diabetes Association Professional Practice Committee for Diabetes. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes-2026. Diabetes Care. 2026;49(suppl 1):S27-S49. doi:10.2337/dc26-S002

Watch one

Read each decision and its explanation.

An adult outpatient with no symptoms has a screening draw. The collection note records 10 hours without food, and the plasma was separated 20 minutes after collection. Both tests come from the same draw.

How do these results classify under the ADA criteria?

TestResultReference intervalFlag
Glucose, fasting134 mg/dL High70–99 mg/dL70–99High
HbA1c6.7 % High4.0–5.6 %4.0–5.6High

Specimen: H 5, L 10, I 1. Fluoride plasma separated 20 minutes after collection; EDTA whole blood for HbA1c

  1. Check the specimen first. The patient fasted 10 hours, and the plasma was separated within 30 minutes.

    A fasting limit applies only to a fasting specimen, and a delayed separation would lower the glucose.

  2. Read the glucose: 134 mg/dL is at or above 126 mg/dL, the diabetes limit for fasting glucose.

    Each result is compared with the ADA limit for its own test.

  3. Read the HbA1c: 6.7% is at or above 6.5%, the diabetes limit for HbA1c.

    The HbA1c has its own limit, and a second test can supply the confirmation.

  4. Apply the confirmation rule. Both tests are in the diabetes range, so each confirms the other.

    Without unequivocal hyperglycemia, diabetes needs two abnormal results, and two different tests can supply them.

The fasting glucose of 134 mg/dL and the HbA1c of 6.7% both meet the ADA limits for diabetes and confirm each other.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Plasma glucose is ordered, and the specimen cannot be centrifuged for 2 hours. Which collection choice best limits in vitro glucose loss?

Correct. Citrate buffer acidifies the sample and suppresses glycolysis promptly, so glucose stays stable until the specimen is centrifuged.

Incorrect. Sodium fluoride does not promptly prevent glucose loss, so clinically important glucose loss can occur during the first hours after collection.

Incorrect. The ice-water slurry is paired with separation within 30 minutes. During a 2-hour delay the cells continue to consume glucose before the plasma is separated.

Hint
  1. Ask which choice stops glycolysis within minutes of collection.
  2. An ice-water slurry protects glucose only when the plasma is separated within 30 minutes.
  3. Ask how fast each additive stops glycolysis.

Review Glucose specimens and methods

Problem 2 of 3 · Hints available

On the second day of a fast, liver glycogen is largely depleted, and plasma glucose remains within its reference interval. Which pathway supplies most of that glucose?

Glycogenolysis breaks down stored glycogen, and muscle lacks glucose-6-phosphatase, so it uses that glucose itself. New glucose made from lactate, glycerol and amino acids comes from gluconeogenesis.

Confused glycogen breakdown with new glucose synthesis

Glycogenolysis breaks down stored glycogen. The liver, which has glucose-6-phosphatase, releases that glucose into blood, and muscle uses its glycogen as fuel for itself. Gluconeogenesis builds new glucose in the liver and kidney from lactate, glycerol, and glucogenic amino acids during fasting. Confusing the two gives the wrong source for fasting blood glucose.

Once glycogen runs low, the liver and kidney build new glucose from lactate, glycerol and amino acids and release it into blood.

Glycolysis breaks glucose down for energy. It uses glucose up and adds none to the blood.

Glycogenesis stores glucose as glycogen. It runs after a meal and takes glucose out of the blood.

Hint
  1. Sort the pathways into those that put glucose into blood and those that take it out.
  2. Ask what the liver can make glucose from once its glycogen is gone.

Review Carbohydrate structure and metabolism

Problem 3 of 3 · On your own

An adult in the emergency department reports weeks of thirst and frequent urination. A random plasma glucose on a properly handled specimen reads 262 mg/dL. How does the result classify under the ADA criteria?

A random glucose of 200 mg/dL or higher with classic symptoms of hyperglycemia is sufficient without a second result.

The confirmation rule applies when hyperglycemia is not unequivocal. Classic symptoms with a random glucose of 200 mg/dL or higher meet the criteria with one result.

A random glucose counts toward diabetes when it is 200 mg/dL or higher with classic symptoms or a hyperglycemic crisis.

Random glucose is not used to classify prediabetes, and 262 mg/dL with symptoms already meets the diabetes criteria.

Review Diabetes and prediabetes criteria

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • A 61-year-old man, MRN 4471026, has a diabetes screen drawn at a satellite clinic at 07:05 after a 12-hour fast.
  • The glucose is in a sodium fluoride tube, sent as unspun whole blood at room temperature. It reaches the laboratory at 11:20.
  • The HbA1c comes from the same draw.
  • Your procedure accepts a diagnostic glucose only from plasma separated within 30 minutes of collection or from a citrate-fluoride-EDTA tube.
  • QC for the run is acceptable.
TestResultReference intervalFlag
Glucose, fasting124 mg/dL High70–99 mg/dL70–99High
HbA1c6.6 % High4.0–5.6 %4.0–5.6High

Specimen: H 8, L 12, I 1. Sodium fluoride tube, unspun for 4 hours 15 minutes at room temperature

Decision 1 of 3

What does the glucose of 124 mg/dL show?

Fluoride does not promptly prevent glucose loss. For the first hours in this unspun tube, the cells continued to use glucose, so the true fasting glucose may be higher than 124 mg/dL.

Released a delayed glucose because fluoride was present

Sodium fluoride inhibits enolase only slowly, so cells in the tube keep consuming glucose for the first hours after collection. Treating a delayed fluoride specimen as protected reports a falsely low glucose, which can hide hyperglycemia or suggest hypoglycemia.

The HbA1c of 6.6% is in the diabetes range and the glucose is below 126 mg/dL, so the two do not agree. An HbA1c also says nothing about the handling of a separate glucose tube.

An unspun fluoride tube can lose glucose during the first hours after collection. The loss can lower a diabetic-range fasting glucose below the 126 mg/dL limit.

Review Glucose specimens and methods

Decision 2 of 3

What do you do with the glucose?

The specimen fails your procedure's handling rule, and the reported value may be below the patient's true glucose.

Released a delayed glucose because fluoride was present

Sodium fluoride inhibits enolase only slowly, so cells in the tube keep consuming glucose for the first hours after collection. Treating a delayed fluoride specimen as protected reports a falsely low glucose, which can hide hyperglycemia or suggest hypoglycemia.

The comment leaves a possibly low number in the chart, where it would be interpreted as prediabetes. Your procedure accepts a diagnostic glucose only from a protected specimen.

The glucose the cells consumed is gone. A repeat measures the same depleted plasma.

A protected fasting specimen gives a glucose that reflects the patient's true value. The reason on the order explains the canceled result.

Review Glucose specimens and methods

Decision 3 of 3

The fasting redraw, separated 15 minutes after collection, reads 133 mg/dL. How do the results classify under the ADA criteria?

That glucose came from a specimen that failed the handling rule and was not reported. The protected redraw replaces it.

Without unequivocal hyperglycemia, one abnormal result needs a second before it confirms diabetes. Here the protected glucose supplies that second result.

Classified diabetes from one unconfirmed abnormal result

Without unequivocal hyperglycemia, the ADA criteria require two abnormal results, from a prompt repeat or a different test. Reading one fasting glucose of 130 mg/dL as established diabetes skips the confirmation rule. A single measurement is open to preanalytic and analytic error.

The HbA1c of 6.6% is at or above 6.5%, and the protected fasting glucose of 133 mg/dL is at or above 126 mg/dL. Two abnormal results from different tests confirm each other.

The oral glucose tolerance test is one of the ADA tests. Two abnormal results from HbA1c and fasting glucose already meet the criteria.

Review Diabetes and prediabetes criteria

The unspun fluoride tube waited over 4 hours at room temperature, and the cells consumed glucose during the first hours of the delay. The first glucose of 124 mg/dL was just below the diabetes limit. The protected redraw of 133 mg/dL and the HbA1c of 6.6% gave two abnormal results.

For reference

Sources checked