Overview
Red cells rely on two glucose metabolic pathways for survival: the Embden-Meyerhoff (glycolytic) pathway, which generates ATP, and the hexose monophosphate (HMP) shunt, which protects the cell from oxidative damage. Deficiency of any enzyme in these pathways — most commonly G6PD or pyruvate kinase — can cause hemolysis.
This lesson also covers extracorpuscular causes of hemolysis, where factors external to the red cell — immune antibodies or mechanical fragmentation — destroy otherwise normal red cells.
Learning Objectives
After this lesson you will be able to…- Describe G6PD deficiency, its clinical presentation and laboratory diagnosis.
- Explain the extracorpuscular causes of hemolysis, including immune hemolytic anemias.
- Describe hemolytic anemia due to red cell fragmentation.
- Interpret Direct and Indirect Coomb's test results in the context of immune hemolysis.
- Identify peripheral smear findings characteristic of oxidant hemolysis and microangiopathic hemolysis.
Clinical Story
Why This MattersA young man started on an antimalarial drug develops sudden fatigue and passes cola-colored urine three days later. The physician suspects an oxidant-triggered hemolytic crisis and orders a peripheral smear and G6PD screening. The lab technologist spots bite cells and blister cells on the smear — a critical clue that guides the diagnosis of G6PD deficiency before the enzyme assay is even back.
Core Concepts
G6PD sends glucose-6-phosphate into the HMP shunt to generate NADPH, which reduces oxidized glutathione (GSSG) to reduced glutathione (GSH). GSH prevents H₂O₂ accumulation and hemoglobin oxidation to methemoglobin. In G6PD deficiency (X-linked recessive), red cells cannot degrade oxidant drugs/chemicals. On oxidant exposure (drugs, chemicals, infections, fava beans), Heinz bodies form and hemolysis follows. Variants are classified Class I–V by severity.
Most patients are asymptomatic until exposed to oxidant stress, then develop acute hemolysis with cola-colored urine, jaundice, and rarely renal shutdown. Smear shows bite cells and blister cells (hemoglobin pushed to one side leaving an empty bleb). Heinz bodies are demonstrated with brilliant cresyl blue supravital stain. The methemoglobin reduction test screens for the enzyme deficiency, but reticulocytes contain trace enzyme even in deficient cells, risking false positives during acute hemolysis. G6PD enzyme assays quantify the deficiency definitively.
Antibody binds red cell antigens, and the antigen-antibody complex is removed by splenic/hepatic macrophages (extravascular hemolysis), forming microspherocytes. Alloimmune anemia occurs with mismatched transfusion (naturally occurring IgM anti-A/anti-B causing intravascular hemolysis) or hemolytic disease of the newborn. Autoimmune hemolytic anemia (AIHA) arises from antibodies against one's own red cells — warm (IgG, 37°C, no complement fixation, extravascular) or cold (IgM, near 0°C, fixes complement, intravascular). Causes may be idiopathic or secondary to SLE, lymphoproliferative disease, infections or drugs.
Intravascular hemolysis from physical trauma to red cells inside narrowed or fibrin-laden vessels, or from mechanical/synthetic heart valves. Causes include mechanical heart valves, renal artery stenosis, large hemangioma, HUS, TTP, DIC and lupus. Smear shows fragmented red cells (schistocytes), anisocytosis, poikilocytosis and nRBC. This is termed microangiopathic hemolytic anemia when due to small vessel disease.
Laboratory Principle
The methemoglobin reduction test relies on the ability of NADPH generated via the HMP shunt to reduce methemoglobin back to hemoglobin. Where G6PD is deficient, NADPH cannot be generated, so methemoglobin remains unreduced — a positive screen. The Direct and Indirect Coomb's tests use anti-human globulin (AHG) to bridge and cross-link antibody or complement-coated red cells, causing visible agglutination when immune sensitization is present.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Brilliant cresyl blue | Supravital stain | Demonstrate Heinz bodies | Room temperature, protect from light |
| Methemoglobin reduction reagent | Screening kit | Screen for G6PD deficiency | As per manufacturer instructions |
| Antihuman globulin (AHG) reagent | Polyspecific/monospecific | Direct/Indirect Coomb's testing | 2–8°C |
Step-by-Step Procedure
Draw EDTA anticoagulated venous blood for smear and CBC. Note recent drug/infection history.
Make a Romanowsky-stained smear; look for bite cells, blister cells, spherocytes, schistocytes or agglutination depending on suspected mechanism.
Perform brilliant cresyl blue supravital staining to demonstrate Heinz bodies if G6PD deficiency is suspected.
Run the methemoglobin reduction screening test for G6PD; confirm with quantitative enzyme assay after the acute hemolytic episode resolves.
Perform Direct Coomb's test on patient red cells and Indirect Coomb's test on patient serum to confirm immune hemolysis.
Flow Diagram
Quality Control
Always include positive and negative controls for Coomb's tests. If the positive control shows no agglutination or the negative control shows agglutination, the test is invalid and must be repeated.
Retest G6PD-deficient patients for confirmation after the acute hemolytic episode has resolved, since reticulocytosis can mask deficiency during acute hemolysis.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Bite/blister cells + Heinz bodies | G6PD deficiency oxidant hemolysis | Confirm with enzyme assay after episode resolves; avoid oxidant drugs |
| Positive Direct Coomb's test | Autoimmune or alloimmune hemolysis (antibody on RBC surface) | Identify antibody specificity; treat underlying cause |
| Schistocytes + thrombocytopenia + fever | Microangiopathic hemolytic anemia (TTP/HUS/DIC) | Urgent work-up; treat as medical emergency |
Common Errors & How to Avoid Them
Cause: Young reticulocytes contain higher enzyme levels, masking true deficiency.
Prevention: Repeat testing 2–3 months after the hemolytic episode resolves.
Cause: Inadequate washing of red cells before adding AHG, leaving residual free globulin that neutralizes the reagent.
Prevention: Wash cells at least 3 times with normal saline and decant completely before adding AHG.
Cause: Cold agglutinins cause red cell clumping that can falsely elevate MCV/MCHC on automated counters.
Prevention: Warm the sample to 37°C before analysis if cold agglutinin disease is suspected.
Laboratory Tips from the Bench
Always ask about recent drug exposure (antimalarials, sulfonamides), infections or fava bean ingestion when G6PD deficiency is suspected.
Schistocytes on a smear should always trigger a platelet count check — thrombocytopenia plus schistocytes points strongly toward TTP/HUS/DIC.
"Warm hugs, cold slaps" — warm antibodies (IgG) cause gentler extravascular hemolysis; cold antibodies (IgM) trigger complement and intravascular hemolysis.
Important Notes
Males are typically affected while females are usually carriers, though homozygous affected females do occur.
As with ELISA testing, hemolysed, lipemic or contaminated sera should never be used for serological or enzyme assays as they cause unreliable results.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgePresents with sudden onset fatigue, yellowing of eyes and dark urine 48 hours after starting primaquine for malaria prophylaxis. No prior history of anemia.
Acute onset hemolysis temporally linked to an oxidant drug (primaquine), with bite/blister cells on smear, is classic for G6PD deficiency-induced oxidant hemolysis.
- →Always ask about recent drug history in unexplained hemolysis.
- →Confirm G6PD deficiency with quantitative assay after the acute episode subsides.
- →Counsel patient to avoid future oxidant drug exposure.
Frequently Asked Questions
Older, more deficient red cells are destroyed first, leaving young reticulocytes with relatively higher enzyme activity, which can produce a false-negative or borderline result.
Intravascular hemolysis destroys red cells within blood vessels, releasing free hemoglobin (causing hemoglobinuria/hemosiderinuria). Extravascular hemolysis occurs when macrophages in the spleen and liver remove antibody-coated or damaged cells from circulation.
Both show schistocytes and thrombocytopenia, so clinical context and coagulation tests (PT, APTT, fibrinogen, D-dimer) are needed to distinguish them — DIC typically shows deranged clotting tests while TTP does not.
Quick Revision
10-Minute ReviewKey Takeaways
- Red cells depend on glycolysis for ATP and the HMP shunt for protection against oxidative stress.
- G6PD deficiency is the commonest red cell enzyme defect causing hemolysis.
- Immune hemolytic anemias occur via alloimmune or autoimmune antibody-mediated destruction.
- Coomb's tests (Direct and Indirect) are essential for diagnosing immune hemolysis.
- Mechanical trauma from heart valves or small vessel disease causes red cell fragmentation (schistocytes).
- Laboratory findings of hemolysis include raised bilirubin, LDH, reticulocytosis and reduced haptoglobin.
Competency Checklist
Track Your MasteryReferences
- NIOS Hematology and Blood Bank Technique — Lesson 20: Hemolytic Anemias Due to Abnormal Red Cell Enzymes.
- Dacie JV, Lewis SM. Practical Haematology. 11th ed.