How should I evaluate and manage a patient with a low hematocrit?

Evaluation and Management of Low Hematocrit

Measure hemoglobin instead of relying on hematocrit alone, obtain a complete blood count with red cell indices, and assess iron status with serum ferritin and transferrin saturation as your initial diagnostic approach. 1

Why Hemoglobin is Preferred Over Hematocrit

• Hemoglobin provides more reproducible results across laboratories with lower within-assessment and between-assessment coefficients of variation compared to hematocrit 1
• Hematocrit is affected by storage time of blood samples (can falsely increase by 2-4% with prolonged storage), while hemoglobin remains stable 1, 2
• Patient-specific variables such as serum glucose can falsely elevate hematocrit but do not affect hemoglobin measurement 1, 2
• Single hematocrit measurements should not be used as an isolated laboratory marker for clinical decision-making, particularly in acute settings 1

Initial Laboratory Evaluation

Order the following tests immediately:

• Complete blood count with white blood cells, hemoglobin, platelets, and red cell indices (MCV, MCH, MCHC, RDW) to assess bone marrow function 1, 2
• Reticulocyte count (absolute or reticulocyte index) to evaluate the appropriateness of bone marrow response to anemia 1, 2
• Serum ferritin and transferrin saturation to assess iron status 1, 2
• C-reactive protein to identify concurrent inflammation 2

Abnormalities in two or more cell lines warrant discussion with a hematologist and likely indicate a bone marrow disorder rather than simple anemia 1

Interpreting Red Cell Indices

• Low MCV (microcytic anemia) suggests iron deficiency, folic acid or vitamin B12 deficiency, or inherited disorders of hemoglobin synthesis 1
• A low reticulocyte count in a patient with adequate iron, folate, and B12 most commonly indicates insufficient erythropoietin production or inflammation in CKD patients 1
• High RDW with normal or low MCV suggests possible coexisting iron deficiency 2

Iron Status Assessment

In patients not on erythropoietic-stimulating agents and without significant menstrual or known blood losses, finding iron deficiency should prompt careful assessment for gastrointestinal bleeding. 1

• Ferritin levels < 25 ng/mL in males and < 11 ng/mL in females predict insufficient iron stores for effective erythropoiesis 1
• Transferrin saturation represents iron available to bone marrow for erythropoiesis 1
• MCV is a late marker of iron deficiency and may not be reliable in all contexts 1

Monitoring in Specific Populations

For patients with chronic kidney disease, screen yearly at minimum for anemia. 1

• More frequent monitoring is indicated in diabetic patients, who have higher anemia prevalence regardless of kidney function level, develop anemia at earlier CKD stages, and have higher comorbidity prevalence 1

Alternative Markers in Acute Settings

In trauma or acute bleeding scenarios, serum lactate or base deficit measurements are more sensitive than hematocrit for estimating and monitoring the extent of bleeding and shock. 1

• Initial hematocrit does not accurately reflect blood loss because patients bleed whole blood, and compensatory fluid shifts from interstitial space require time 1
• Resuscitative measures (intravenous fluids, red cell concentrates) confound hematocrit interpretation 1
• Serial hematocrit measurements have limited sensitivity (0.5) for detecting traumatic hemorrhage requiring surgical intervention 1

Common Pitfalls to Avoid

• Do not use a single hematocrit measurement to guide major clinical decisions, particularly regarding bleeding or transfusion 1
• Do not assume anemia of CKD is solely due to erythropoietin deficiency—patients may have various underlying conditions contributing to anemia 1
• Do not overlook the need for hematology consultation when multiple cell lines are affected 1
• Remember that the anemia of CKD is typically normochromic and normocytic, indistinguishable from anemia associated with other chronic conditions 1