Difference Between PCV and HCT
Packed cell volume (PCV) and hematocrit (HCT) are essentially the same measurement—both represent the percentage of whole blood volume occupied by red blood cells—but they differ in how they are measured. 1
Measurement Methods
- PCV is measured directly by centrifuging a blood sample in a capillary tube and physically measuring the proportion of packed red cells to total blood volume 1, 2
- HCT is calculated indirectly by automated cell counters using the formula: mean corpuscular volume (MCV) × red blood cell count = HCT 3, 1
- Both methods should yield similar results under ideal conditions, but automated HCT calculation is subject to multiple sources of error that do not affect direct PCV measurement 1
Key Technical Differences
Storage Stability
- HCT becomes unreliable after prolonged storage because MCV remains stable for only 8 hours at room temperature or 24 hours when refrigerated 3, 1
- After these time periods, MCV artificially increases, causing calculated HCT to falsely rise by 2-4% without any true change in red cell mass 3, 1
- In contrast, direct PCV measurement by centrifugation can be affected by plasma trapping and cell shrinkage artifacts 4
Hyperglycemia Effects
- Elevated glucose falsely elevates MCV, leading to spuriously high calculated HCT values 3, 1
- Direct PCV measurement is not affected by hyperglycemia in the same way 1
Analytical Precision
- HCT has 2-3 times greater variability across automated analyzers compared to hemoglobin measurement, reflected in higher coefficients of variation 3, 1
- The within-run and between-run coefficient of variation for HCT is double and triple that of hemoglobin, respectively 3
Clinical Implications
Which to Use
- The NKF-K/DOQI guidelines strongly recommend using hemoglobin rather than either HCT or PCV as the primary laboratory parameter for clinical decision-making 3, 1
- This recommendation is based on hemoglobin's superior stability, accuracy, and lower analytical variability 3, 1
Common Pitfalls
- Do not rely on HCT from samples transported long distances or stored for prolonged periods, as false elevations are common 1
- Do not use HCT or PCV alone to guide transfusion decisions in acute bleeding, as volume shifts from resuscitation fluids can mask the true red cell mass 5, 1
- A single HCT or PCV measurement is unreliable for detecting acute hemorrhage; serial measurements are required 5, 1
- In diabetic patients, calculated HCT may be falsely elevated due to hyperglycemia-induced MCV changes 3, 1
When Values Diverge
- Both HCT and PCV reflect the ratio of red cell volume to plasma volume, so they can appear normal despite true anemia if plasma volume is also reduced (relative polycythemia) 6
- Conversely, true polycythemia can be masked by increased plasma volume (inapparent polycythemia), causing both HCT and PCV to appear falsely normal 6