What are the next steps if a fasting lab shows a mildly elevated Hematocrit (HCT) and a mildly low Mean Corpuscular Hemoglobin Concentration (MCHC) with all other results being normal?

Evaluation and Management of Mildly Elevated Hematocrit with Low MCHC

When a fasting lab shows mildly elevated hematocrit (HCT) with low mean corpuscular hemoglobin concentration (MCHC) and all other results normal, the most likely cause is dehydration with early iron deficiency, requiring rehydration followed by repeat testing and possible iron studies.

Understanding the Laboratory Abnormalities

• Elevated hematocrit indicates a relative increase in the proportion of red blood cells in blood volume, commonly due to dehydration (relative polycythemia) 1
• Low MCHC suggests decreased hemoglobin concentration within red blood cells, most commonly seen in iron deficiency 1
• The combination of normal MCV with low MCHC typically represents early iron deficiency before microcytosis develops 1
• These findings together often represent a combination of dehydration (causing elevated HCT) and early iron deficiency (causing low MCHC) 1

Diagnostic Algorithm

Step 1: Assess for Dehydration

• Evaluate clinical signs of dehydration (dry mucous membranes, decreased skin turgor, orthostatic hypotension) 1
• Review patient's fluid intake, losses (vomiting, diarrhea, excessive sweating), and medication use (diuretics) 1
• Consider fasting status impact on hydration (patient was fasting for labs) 1

Step 2: Repeat CBC After Hydration

• Ensure adequate hydration and repeat CBC to determine if hematocrit normalizes 1
• If hematocrit normalizes but MCHC remains low, dehydration was contributing to the elevated HCT 1
• If both normalize, the abnormalities were likely due to dehydration alone 1

Step 3: Evaluate Iron Status

• If MCHC remains low after hydration, order iron studies: serum ferritin, iron, total iron binding capacity (TIBC), and transferrin saturation 1
• Low ferritin and/or low transferrin saturation would confirm iron deficiency 1

Management Recommendations

For Dehydration

• Increase oral fluid intake to 2-3 liters daily unless contraindicated 1
• Address any underlying causes of fluid loss or decreased intake 1
• Consider IV fluids if dehydration is severe or oral intake is not possible 1

For Suspected Iron Deficiency

• If iron deficiency is confirmed, initiate oral iron supplementation (ferrous sulfate 325 mg daily or every other day) 2
• Consider intermittent dosing (every other day) which may improve tolerance with similar efficacy 2
• Investigate potential causes of iron deficiency (blood loss, malabsorption, poor dietary intake) 1

Follow-up Recommendations

• Repeat CBC after ensuring adequate hydration (typically within 1-2 weeks) 1
• If hematocrit normalizes but MCHC remains low, proceed with iron studies 1
• If both parameters normalize with hydration alone, consider routine follow-up in 3-6 months 1
• If hematocrit remains elevated despite adequate hydration, consider evaluation for true polycythemia 1

Important Considerations

• Single Hct measurements should not be used as an isolated marker for bleeding or other conditions 3
• Automated cell counters should be used for measuring RBC indices as they provide more standardized results 3
• MCHC values can occasionally be falsely elevated or decreased due to technical factors or sample handling 4, 5
• Hemoglobin is generally more stable than hematocrit when blood samples are stored, so consider hemoglobin values in interpretation 3

By following this systematic approach, you can efficiently determine whether the laboratory abnormalities represent a clinically significant condition requiring intervention or are simply due to dehydration that will resolve with adequate fluid intake.