HemoglobinHb
Oxygen-carrying protein in red blood cells; the central screen for anemia and erythrocytosis.
What it measures
Hemoglobin is a tetramer (2α/2β in adults, HbA) that binds O₂ cooperatively. Synthesis depends on iron (heme), B12/folate (DNA synthesis in erythroblasts), and EPO from the kidney sensing tissue hypoxia.
Why we order it
Diagnose anemia, monitor erythropoietic therapy, detect erythrocytosis, and trigger transfusion thresholds (typically 7 g/dL in stable adults, 8 in cardiac disease — TRICC/TRISS).
Interpretation by lens
Use Hb plus MCV and RDW as a first triage. WHO anemia thresholds: men <13.0, non-pregnant women <12.0, pregnant women <11.0 g/dL.
Functional optimal is mid-range (~14 g/dL women, 15 g/dL men). Both extremes deserve a workup — the upper end may reflect smoking, OSA, or testosterone overuse before it reflects polycythemia vera.
Hb is a poor measure of oxygen delivery; consider 2,3-BPG, p50, and tissue O₂ extraction in critical care research. Hb glycation index (HGI) modulates HbA1c interpretation independent of glycemia.
Causes of abnormal values
- ↑Hemoconcentration (dehydration, diuretics)
- ↑Smoking and chronic hypoxia (COPD, OSA, high altitude)
- ↑Polycythemia vera (with low EPO and JAK2 V617F)
- ↑Secondary erythrocytosis (EPO-secreting tumors, androgen use, testosterone replacement)
- ↓Iron deficiency (microcytic, low ferritin)
- ↓B12 / folate deficiency (macrocytic)
- ↓Anemia of chronic disease / inflammation (normocytic, high ferritin, low TSAT)
- ↓Hemolysis (high LDH, low haptoglobin, high indirect bilirubin)
- ↓Bone-marrow failure or infiltration
- ↓Acute or occult blood loss
Pattern recognition
Iron-deficiency anemia until proven otherwise. Investigate GI loss in adults; menstrual loss in premenopausal women.
B12 or folate deficiency. Order B12, folate, MMA, homocysteine. Don't miss drug-induced macrocytosis (hydroxyurea, methotrexate, alcohol).
Hemolysis. Coombs, peripheral smear, G6PD if suggested by ethnicity/triggers.
Pre-analytic & interpretation traps
- !Tourniquet >1 min falsely elevates Hb (hemoconcentration) by up to 10%.
- !Severe leukocytosis (>50 ×10⁹/L) or hyperlipidemia falsely elevates spectrophotometric Hb readings.
- !Pregnancy hemodilution is physiologic; do not over-treat.
Logical next-step labs
What the data says
Primary literature
- Restrictive vs liberal transfusion (TRICC trial) — Hébert PC et al., NEJM 1999
- WHO haemoglobin thresholds for anaemia (2024 update) — WHO