What CBC abnormality is associated with severe hyperglycemia?

CBC Abnormality in Severe Hyperglycemia: Falsely Elevated MCV

Patients with severe hyperglycemia (such as a glucose of 817 mg/dL) will demonstrate a falsely elevated Mean Corpuscular Volume (MCV) on their CBC, which can help identify hyperglycemic patients even before glucose results are available.

Mechanism of Falsely Elevated MCV in Hyperglycemia

The mechanism behind this phenomenon is osmotic in nature:

1. Osmotic Effect: When blood with extremely high glucose concentration is placed in the isotonic diluent of automated cell counters, the red blood cells (RBCs) rapidly absorb water due to the osmotic gradient 1, 2.

2. Cell Swelling: This rapid water influx causes the RBCs to swell during measurement, resulting in an artificially increased MCV reading 1.

3. Temperature Dependence: This phenomenon is temperature-dependent and rapidly reversible once the glucose concentration normalizes 1.

4. Proportional Relationship: The degree of MCV elevation is proportional to the glucose concentration - the higher the glucose level, the more pronounced the MCV elevation 2.

Associated CBC Findings

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

• Hematocrit: Falsely elevated due to the increased cell volume 2
• MCHC (Mean Corpuscular Hemoglobin Concentration): Falsely decreased due to the dilutional effect of water entering the cells 1
• MCH (Mean Corpuscular Hemoglobin): Generally remains normal 1
• RDW (Red Cell Distribution Width): May be negatively correlated with poor glycemic control 3

Clinical Significance

This laboratory finding is clinically significant for several reasons:

• It can serve as an early indicator of severe hyperglycemia before glucose results are available
• It may lead to misdiagnosis of macrocytic anemia if the hyperglycemia is not recognized
• The degree of MCV elevation may provide a clue to the severity of hyperglycemia
• The phenomenon is reversible with correction of hyperglycemia 2

Confirmation and Correction

Blood smear examination will not corroborate the electronically measured MCV elevation, as the cells appear normal on microscopic examination 1.

To obtain accurate MCV values in patients with severe hyperglycemia:

• Predilute the blood sample in isotonic medium
• Allow 10-15 minutes for equilibration before measurement 1
• Alternatively, repeat the CBC after glucose normalization

Clinical Context

Severe hyperglycemia (>600 mg/dL) as seen in this case is consistent with hyperosmolar hyperglycemic state (HHS), which typically presents with:

• Extreme hyperglycemia
• Serum osmolality >320 mOsm/kg
• Severe dehydration
• Often altered mental status 4

Important Considerations

1. This phenomenon is more pronounced with certain automated cell counters (e.g., more with Coulter S-Plus than with Coulter S) due to differences in incubation time prior to measurement 2.

2. The ratio of change in serum sodium level over change in serum glucose concentration is approximately -1.50 mmol/L per 100 mg/dL increase in glucose 5.

3. In patients with renal failure, the absence of osmotic diuresis modifies the effects of hyperglycemia on body fluids, potentially exacerbating the MCV abnormality 5.

4. The falsely elevated MCV should normalize after treatment of hyperglycemia, and persistence of macrocytosis after glucose normalization should prompt investigation for other causes.