Hemolysis

The Clinical Impact of Hemolyzed Blood Samples

Hemolysis is the rupture of red blood cells (RBCs) with consequent release of intracellular contents (e.g., hemoglobin, potassium, lactate dehydrogenase) into the plasma or serum. In phlebotomy, a hemolyzed specimen is the single most common cause of sample rejection by clinical laboratories.^[1] Unrecognized hemolysis can lead to falsely elevated potassium (pseudohyperkalemia), skewed enzyme assays, and inaccurate blood counts, which, in turn, may trigger dangerous medical decisions such as unnecessary emergency treatments or missed diagnoses.^[2] Understanding how hemolysis occurs and how to prevent it is therefore a high-yield topic for both the national certification exam and daily clinical practice.

Essential Terminology for Hemolysis in Phlebotomy

• Hemolysis – The breakdown of RBC membranes causing leakage of hemoglobin and other intracellular components into the surrounding fluid.
• Hemolyzed Specimen – A blood sample visually characterized by a pink-to-red tint in the plasma or serum (versus the normal straw-yellow or clear appearance).
• In Vitro Hemolysis – Hemolysis that occurs during or after venipuncture (i.e., as a result of technique or handling), which is the focus of phlebotomy prevention.
• Pseudohyperkalemia – A falsely elevated potassium reading due to release of potassium from lysed RBCs; can mimic a true hyperkalemic emergency.
• Hemolysis (H) Index – A spectrophotometric measurement reported by many clinical analyzers to quantify free hemoglobin in a specimen. Samples exceeding a lab-specific threshold (e.g., H > 100 mg/dL) are typically rejected.^[3]

Preventing Hemolysis: Causes and Best Practices

Most hemolysis in phlebotomy is preventable. The following principles focus on the physical forces that rupture RBCs during collection and processing.

Common Causes of In Vitro Hemolysis

• Needle gauge too small – Using a fine-gauge needle (e.g., 23G or smaller) for a large vein creates excessive shear stress as blood accelerates through the narrow lumen.^[4]
• Prolonged tourniquet time – Tourniquet left on longer than 1 minute causes stasis and hemoconcentration, making RBCs more fragile.
• Excessive probing or repeated venipuncture attempts – Traumatic puncture causes cells to be damaged at the site of entry.
• Forceful mixing or shaking of tubes – Agitation mechanical stress lyses RBCs, especially with partial clotting tubes (serum separator tubes).
• Moist or partially clotted collection tubes – Microclots within the tube can entrap RBCs and shear them during mixing or centrifugation.
• Improper use of syringe-to-tube transfer – Forcing blood into a tube with positive pressure (e.g., pushing on the plunger) creates high shear forces.
• Centrifugation at too high a speed or for too long – Mechanical stress from centrifugal force can cause additional lysis in already fragile samples.

Step-by-Step Prevention Strategy

1. Select the appropriate vein and needle gauge: Use 21–22G for antecubital veins; reserve 23G butterfly needles for small or fragile veins, and follow with slow, steady draw.
2. Release the tourniquet within 1 minute of vein entry, or earlier if blood flow is adequate.
3. Anchor the vein and perform a smooth, single-puncture technique to minimize tissue trauma.
4. Fill tubes in the correct order (blood culture → citrate → serum → heparin → EDTA → glycolysis inhibitor) to avoid carryover of additives that may cause hemolysis.
5. Gently invert tubes 5–8 times (not vigorous shaking) to mix additive and blood.
6. Allow serum tubes to clot completely (typically 30 minutes) before centrifugation.
7. Centrifuge according to manufacturer instructions (e.g., 3000–3500 RPM for 10 minutes for serum/clot tubes).

Detecting Hemolysis: Visual and Laboratory Indicators

• Visual inspection (most common): Centrifuged serum or plasma appears pink or red instead of the normal straw-yellow color. Mild hemolysis may be detected only by the lab’s H index.
• Elevated serum/plasma potassium – Often one of the first laboratory clues (may exceed 6.0–7.0 mmol/L).^[2]
• Increased lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) – Both released intra-RBC.
• Decreased haptoglobin – Haptoglobin binds free hemoglobin; levels drop when hemolysis is systemic, but in vitro hemolysis does not usually affect haptoglobin.

Evaluating Hemolysis: H Index and Specimen Rejection

• Visual check after centrifugation: Compare the supernatant against a color chart or the lab’s H index standards. Even slight pink coloration may be unacceptable for certain tests (e.g., potassium, cardiac troponin).
• Do not rely solely on visual inspection – Some labs require spectrophotometric H index for accurate quantification. Many modern analyzers automatically flag hemolyzed samples.
• Know your facility’s rejection criteria: Most clinical laboratories reject specimens with a hemolysis index above a specific threshold (e.g., H index > 100–200 mg/dL for routine chemistry).^[3]
• In a patient with true hyperkalemia (hemolysis suspected vs. real), check for ECG changes, perform a point-of-care potassium (non-hemolyzed sample via arterial line or fresh venipuncture), and evaluate clinical signs.

Managing Hemolyzed Specimens: Recollection and Patient Communication

• When hemolysis is detected (by visual inspection or instrument flag) – the specimen is rejected. The phlebotomist must recollect the sample to avoid inaccurate results.
• Explain the need for a repeat draw to the patient briefly and professionally; do not blame technique or equipment.
• Use a fresh venipuncture site for the new draw. Avoid using a line that was previously cannulated.
• If the patient has a difficult venous access, consider using a butterfly needle with a low-volume collection tube and a slower draw rate, or consult a nurse/IV team for alternative access.
• Document the incident (including possible cause of hemolysis) in the patient’s chart according to facility policy.

Clinical Risks and Safety Measures for Hemolysis

• Clinical risk: Reporting a falsely elevated potassium can lead to unnecessary administration of Kayexalate, intravenous calcium, insulin/dextrose, or urgent dialysis—all of which carry risks to the patient.
• Repeat venipunctures increase patient discomfort, risk of hematoma, and potential for infection.
• Do not attempt to “fix” a hemolyzed sample by adding water, saline, or other diluents; this violates CLSI and OSHA best practices and can introduce further errors.^[4]
• Sharps safety: Always use a new needle for the repeat draw; never recap or attempt to reuse a needle.

Exam-Relevant Strategies and Common Pitfalls

• Top test question: “What is the most common cause of sample rejection in the lab?” – Answer: Hemolysis.
• Remember “PHL” for the main causes: P – prolonged tourniquet, H – heavy agitation/shaking, L – small lumen (needle gauge too small).
• Pseudohyperkalemia: Hemolyzed sample → high potassium → but no ECG changes and patient asymptomatic. Remember that hemolysis falsely elevates potassium.
• Visual comparison: A pink/red supernatant = hemolysis. A yellow supernatant = normal.
• Syringe transfer: Never push blood through a needle into the tube; allow the vacuum to fill the tube or use a blood transfer device.
• Order of draw: If you don’t follow the correct order, additives (e.g., potassium EDTA from a lavender-top tube) can contaminate subsequent serum tubes and cause hemolysis or erroneously high potassium.^[4]
• Practice scenario: A 22G butterfly is used on a large antecubital vein → high risk of hemolysis due to shear stress. Better to use 21G straight needle or a 23G with slow draw.

References

1. CLSI. Procedures for the Collection of Diagnostic Blood Specimens by Venipuncture; Approved Standard—6th Edition. CLSI document H3-A6. Clinical and Laboratory Standards Institute; 2007. https://webstore.ansi.org/preview-pages/clsi/preview_h03-a6.pdf?srsltid=AfmBOoqU3LCq7e3k1m2Zbw0oGCODLGAI1GSINMHLYVK3vZcELSJdcXcv
2. Lippi G, Cervellin G, Mattiuzzi C. Critical review and meta-analysis of the impact of hemolysis on laboratory test results. Clin Chim Acta. 2021;516:108-117. https://doi.org/10.1016/j.cca.2021.01.024
3. CLSI. Hemolysis, Icterus, and Lipemia/Turbidity Indices as Indicators of Interference in Clinical Laboratory Tests; Approved Guideline—2nd Edition. CLSI document C56-A. Clinical and Laboratory Standards Institute; 2012. https://clsi.org/standards/products/clinical-chemistry-and-toxicology/documents/c56/
4. McCall RE, Tankersley CM. Phlebotomy Essentials. 8th ed. Wolters Kluwer; 2024. https://www.jblearning.com/catalog/productdetails/9781284263480?srsltid=AfmBOop_BhKk2_4LLIq-I0zh7KLBowc8EhICeLprq7xYzFZCSdQXiUdY