Laboratory Evaluation for Suspected Hemolysis
Order a complete blood count with reticulocyte count, peripheral blood smear, lactate dehydrogenase (LDH), indirect bilirubin, and haptoglobin as your initial hemolysis panel 1.
First-Line Laboratory Tests
The diagnosis of hemolysis is established through a combination of tests that demonstrate both red blood cell destruction and the body's compensatory response:
Essential Initial Tests
- Complete Blood Count (CBC): Documents anemia and provides baseline hemoglobin/hematocrit
- Reticulocyte Count: Elevated reticulocytes (>2%) indicate bone marrow compensation for hemolysis 1
- Peripheral Blood Smear: Critical for identifying schistocytes (microangiopathic hemolysis), spherocytes (hereditary spherocytosis, autoimmune hemolysis), sickle cells, or other morphologic abnormalities 2, 1
- Lactate Dehydrogenase (LDH): Elevated due to release from lysed red blood cells 1
- Indirect (Unconjugated) Bilirubin: Increased from heme catabolism 1
- Haptoglobin: Decreased or absent as it binds free hemoglobin (note: only useful after 6 months of age) 3, 1
Important Caveats
The peripheral blood smear is non-negotiable—schistocytes may be initially inapparent but are crucial for diagnosing thrombotic microangiopathies like TTP 2. In TTP specifically, thrombocytopenia is typically severe (<30 × 10⁹/L), and the direct antiglobulin test is nearly always negative with normal coagulation studies 2.
Second-Line Tests Based on Clinical Context
If Immune-Mediated Hemolysis Suspected
- Direct Antiglobulin Test (DAT/Coombs): Detects antibodies or complement on red blood cell surface
- Indirect Antiglobulin Test: Identifies circulating antibodies
If Microangiopathic Hemolysis/TMA Suspected
- ADAMTS13 Activity: Must be <10% to confirm TTP; draw sample before plasma exchange 2
- ADAMTS13 Antibodies: Distinguishes immune TTP from other forms 2
- Complement Studies: For complement-mediated TMA (functional assays like modified Ham test or HMEC-1 C5b-9 deposition) 4
- Shiga Toxin Testing: To exclude STEC-HUS 4
If Hereditary Hemolysis Suspected
- Glucose-6-Phosphate Dehydrogenase (G6PD) Screening: Common enzymatic deficiency 1
- Hemoglobin Electrophoresis: Identifies sickle cell disease and thalassemias 1
- Osmotic Fragility Test: For hereditary spherocytosis 1
- Red Blood Cell Enzyme Assays: Pyruvate kinase activity if other tests negative (requires reticulocyte count interpretation and should be performed >50 days post-transfusion) 3
Specialized Tests
- Plasma-Free Hemoglobin: Indicates intravascular hemolysis severity; particularly monitored in mechanical circulatory support 5, 6
- Erythrocyte Adenylate Kinase (EAK): Highly sensitive (96%) and specific (97%) red cell-specific enzyme for confirming hemolysis 7
- Urinalysis: Hemoglobinuria suggests severe intravascular hemolysis 2
Diagnostic Algorithm
- Establish hemolysis exists: Reticulocytosis + elevated LDH + elevated indirect bilirubin + decreased haptoglobin + anemia
- Examine peripheral smear: Determines intravascular vs. extravascular and suggests etiology
- Rule out acquired causes first: DAT, infection screening, drug history
- If microangiopathic features present: Urgent ADAMTS13 activity, complement studies, Shiga toxin
- If DAT negative with spherocytes: Consider hereditary spherocytosis
- If oxidative stress history: G6PD screening
- If chronic hemolysis with family history: Hemoglobin electrophoresis, enzyme assays
Critical Pitfalls
- Timing matters: Haptoglobin is unreliable in infants <6 months; enzyme assays are unreliable within 50 days of transfusion 3
- Don't delay treatment for test results: In suspected TTP, start plasma exchange and corticosteroids before ADAMTS13 results return 2
- Reticulocyte count affects enzyme assays: Young red cells have higher enzyme activity; compare to age-matched controls or calculate enzyme ratios 3
- Normal platelet count doesn't exclude TMA: Though typically severe in TTP, higher counts don't rule out diagnosis 2