What is the use of Hemoglobin (Hgb) and Hematocrit (Hct) in clinical practice?

Clinical Use of Hemoglobin (Hgb) and Hematocrit (Hct)

Hemoglobin measurement is the preferred method for assessing anemia due to its greater accuracy and stability compared to hematocrit, which is more susceptible to variations from sample storage conditions and hydration status. 1

Normal Values and Diagnostic Thresholds

Hemoglobin and hematocrit are essential laboratory parameters used to diagnose anemia and polycythemia. The World Health Organization defines anemia based on the following thresholds:

Population	Hemoglobin (g/dL)	Hematocrit (%)
Adult males	<13.0	<39
Non-pregnant women	<12.0	<36
Pregnant women	<11.0	<33
Children (varies by age)	Variable	Variable

Normal values vary by age and sex, as shown in this table:

Age/Gender	Hemoglobin (g/dL)	Hematocrit (%)
Birth	16.5 ± 3.0	51 ± 9
Adult male/post-menopausal female	15.5 ± 2.0	47 ± 6
Menstruating female	14.0 ± 2.0	41 ± 5

2, 1

Advantages of Hemoglobin Over Hematocrit

Hemoglobin is superior to hematocrit for several reasons:

• Stability: Hemoglobin remains stable when blood samples are stored at room temperature, while hematocrit (calculated from MCV) increases by 2-4% after 8 hours at room temperature 2
• Accuracy: Hemoglobin has lower within-run and between-run coefficients of variation (CV) compared to hematocrit (one-half and one-third those of hematocrit, respectively) 2
• Reliability: Hemoglobin is not affected by hyperglycemia, which falsely elevates MCV and calculated hematocrit 2
• Consistency: Hemoglobin measurements show less variability across automated analyzers than hematocrit calculations 2, 3

Clinical Applications

1. Anemia Detection and Monitoring

• Initial diagnosis of anemia based on WHO thresholds
• Monitoring response to treatment (iron supplementation, erythropoietin therapy)
• In chronic kidney disease (CKD), the target range for hemoglobin should be 11-12 g/dL (hematocrit 33-36%) when using erythropoietin therapy 2, 4
• Higher hemoglobin targets (>13 g/dL) in CKD patients increase risk of death, myocardial infarction, and stroke 4

2. Blood Loss Detection

• Serial measurements increase sensitivity for detecting blood loss
• Initial normal values may mask early-phase bleeding
• Particularly valuable in critically ill patients and perioperative settings 1

3. Polycythemia Evaluation

• Elevated hematocrit is a key diagnostic criterion for polycythemia vera
• Treatment targets: <45% in men and <42% in women 1
• Relative polycythemia can occur with plasma volume depletion (dehydration) 1

4. Transfusion Decisions

• In stable hospitalized patients, transfusion is generally recommended when hemoglobin falls below 7-8 g/dL (hematocrit 20-24%) 5
• In surgical patients over age 40, maintaining hemoglobin >10 g/dL (hematocrit >30%) may be appropriate, especially with risk factors for silent myocardial ischemia 6

Important Clinical Considerations

Interpretation Pitfalls

• Hydration status: Dehydration elevates hematocrit without changing actual red cell mass 1
• Timing of collection: For hemodialysis patients, samples should be collected pre-dialysis 2
• Sample storage: Extended storage time falsely elevates hematocrit but not hemoglobin 2
• Population norms: Statistical benchmarks may not indicate anemia in every individual; clinical context is essential 2

Clinical Outcomes Related to Hgb/Hct Levels

• In CKD patients, hemoglobin levels of 11-12 g/dL are associated with:

  • Reduced mortality compared to higher targets (>13 g/dL) 4, 7
  • Decreased risk of hospitalization (7-58% reduction) 8
  • Partial regression of left ventricular hypertrophy 2
  • Decreased angina in patients with progressive CKD 2

• Targeting hemoglobin >13 g/dL in CKD patients is associated with:

  • Increased mortality 4, 7
  • Higher risk of myocardial infarction and stroke 4
  • Increased risk of vascular access thrombosis 4

Algorithmic Approach to Using Hgb/Hct

1. Obtain baseline measurement

   • Use hemoglobin as primary parameter when possible
   • Collect pre-dialysis samples in hemodialysis patients

2. Interpret results based on age/sex-specific reference ranges

   • Consider clinical context (pregnancy, altitude, comorbidities)

3. If abnormal:

   • Evaluate RBC indices (MCV, MCHC, RDW)
   • Order appropriate additional tests (reticulocyte count, iron studies, B12/folate)

4. For monitoring:

   • Use serial measurements to detect trends
   • In CKD patients on erythropoietin therapy, target Hgb 11-12 g/dL (Hct 33-36%)
   • In acute blood loss, repeat measurements to detect ongoing losses

5. For transfusion decisions:

   • Use restrictive strategy (Hgb 7-8 g/dL) in stable patients
   • Consider higher thresholds in patients with cardiovascular disease or over age 40