Calculation of Hematocrit from RBC Count and MCV
The correct hematocrit (HCT) is 31.5% (option B).
Formula and Calculation
Hematocrit can be calculated using the following formula:
HCT = RBC × MCV
Where:
- HCT = Hematocrit (%)
- RBC = Red Blood Cell count (in millions/μL)
- MCV = Mean Corpuscular Volume (fL)
Applying the formula with the given values:
- RBC = 3.5 × 10^6/μL
- MCV = 90 fL
HCT = 3.5 × 10^6/μL × 90 fL HCT = 315 × 10^6 fL/μL HCT = 31.5%
Explanation of the Relationship
This calculation is based on the fundamental relationship between red blood cell parameters. The hematocrit represents the percentage of blood volume occupied by red blood cells 1. Since each red blood cell has a specific volume (measured by MCV), the total volume occupied by all red blood cells can be calculated by multiplying the number of cells by their average volume.
Clinical Context
The calculated hematocrit of 31.5% is below the normal range for adults:
- Adult males: 39-47%
- Non-pregnant females: 36-41% 1
This indicates anemia in this patient, which is consistent with the hemoglobin value of 9.2 g/dL (also below normal range).
Verification Methods
In clinical practice, hematocrit can be measured directly or calculated:
- Direct measurement: Centrifugation of whole blood to separate and measure the packed cell volume
- Calculated method: Using the RBC count and MCV as demonstrated above
Modern automated hematology analyzers typically use the calculated method, which has been shown to be reliable and accurate 2. The calculated hematocrit is considered more stable than direct measurements, especially when samples are stored for periods of time before analysis 3, 4.
Common Pitfalls to Avoid
- Confusing units: Ensure RBC count is in millions/μL and MCV is in fL
- Failing to convert to percentage: Remember to express the final answer as a percentage
- Not accounting for MCV changes: MCV can decrease during conditions like hemodialysis (by approximately 1.5%), which may affect hematocrit calculations 4
- Relying solely on hematocrit: Hemoglobin measurement is more stable than hematocrit and less affected by storage conditions 1
By understanding this calculation, clinicians can better interpret complete blood count results and identify discrepancies that might indicate laboratory error or specific pathological conditions.