What CBC abnormality is associated with hyperglycemia?

CBC Abnormality in Severe Hyperglycemia: Falsely Elevated MCV

In patients with severe hyperglycemia (such as a glucose of 817 mg/dL), the most significant CBC abnormality is a falsely elevated mean corpuscular volume (MCV) without true macrocytosis. This laboratory finding can help identify patients with undiagnosed severe hyperglycemia when reviewing CBC results.

Mechanism of Falsely Elevated MCV in Hyperglycemia

The mechanism behind this phenomenon is osmotic in nature:

1. Osmotic Effect: Extremely high glucose levels create a hyperosmolar environment in the blood 1

2. Red Blood Cell Swelling: When the blood sample is diluted in the electronic cell counter's isotonic medium, water rapidly diffuses into the glucose-laden red blood cells 1, 2

3. Artificial Volume Increase: This osmotic swelling causes the electronic analyzer to measure an artificially increased cell volume 2

4. Temperature Dependent: The phenomenon is temperature dependent and rapidly reversible 1

Associated Laboratory Findings

Along with the falsely elevated MCV, other CBC parameters may show abnormalities:

• Falsely elevated hematocrit due to the increased cell volume 1, 2
• Falsely decreased MCHC (mean corpuscular hemoglobin concentration) 1
• Normal MCH (mean corpuscular hemoglobin) as the hemoglobin content remains unchanged 1
• Blood smear examination will not show true macrocytosis, creating a discrepancy between the electronic MCV and microscopic appearance 1, 2

Clinical Significance

This laboratory finding is particularly important because:

• It can serve as a clue to undiagnosed severe hyperglycemia when reviewing CBC results
• The degree of MCV elevation is proportional to the glucose concentration 2
• The effect is more pronounced with some analyzers than others (e.g., more with Coulter S-Plus than with Coulter S) 2
• The abnormality resolves after normalization of blood glucose levels 2

How to Confirm and Correct the Measurement

If a falsely elevated MCV is suspected due to hyperglycemia:

1. Pre-dilution method: Dilute the blood sample in isotonic medium and allow 10-15 minutes for equilibration before measurement 1

2. Re-check after glucose normalization: The MCV will normalize after treatment of hyperglycemia 2

3. Blood smear examination: This will show normal-sized RBCs despite the electronically elevated MCV 1

Clinical Context

This laboratory finding is particularly relevant in the emergency department and critical care settings where patients may present with severe, undiagnosed hyperglycemia. Recognizing this pattern can help identify patients with hyperglycemic crisis who require immediate intervention to reduce morbidity and mortality.

Remember that hyperglycemia during critical illness is associated with high levels of mortality and morbidity 3, and controlling hyperglycemia improves overall outcomes 4. Therefore, recognizing this CBC abnormality can contribute to earlier diagnosis and treatment.