Hemolyzed specimens are the single most common cause of specimen rejection in clinical laboratories. A hemolyzed sample means red blood cells have ruptured and released hemoglobin into the serum or plasma, contaminating the specimen and rendering many tests unreliable. Better news: hemolysis is almost entirely preventable with proper technique. This guide covers the eight most common causes of hemolysis and how to prevent each one.
What Is Hemolysis and Why Does It Matter?
Hemolysis is the rupture of red blood cell membranes, releasing hemoglobin and intracellular contents (potassium, LDH, ALT, AST, and others) into the serum or plasma. A hemolyzed specimen appears pink, red, or brown in color, depending on the degree of hemolysis.
Hemolysis affects the accuracy of these tests:
- Potassium (K+): Falsely elevated (RBCs contain ~150 mEq/L potassium vs. serum ~3.5-5.0 mEq/L)
- LDH (lactate dehydrogenase): Falsely elevated (high in RBCs)
- ALT, AST (liver enzymes): Falsely elevated (present in RBCs)
- Hemoglobin/hematocrit: Falsely interpreted
- Bilirubin: Spectrophotometry interference (hemoglobin absorbs light at 540 nm)
- Magnesium: Falsely elevated
- Albumin and total protein: May be affected by assay method
The clinical impact is real: a falsely elevated potassium might trigger an emergency intervention for a patient who doesn't have hyperkalemia. This is not a quality-of-care issue to brush aside.
The 8 Most Common Causes of Hemolysis
Cause 1: Wrong Needle Gauge (Too Small)
What happens: A 23G or smaller needle creates excessive pressure and shear stress on red blood cells as they are forced through the narrow bore at high velocity.
Prevention:
- Use appropriate gauge for the draw:
- Adults, routine draw: 21G straight needle (standard)
- Adults, high-volume draw (10+ tubes): 20G or 21G
- Difficult veins: 23G butterfly (acceptable because needle is shorter and insertion angle is more controlled)
- Pediatric: 23G-25G butterfly is standard and acceptable
- Avoid using 25G or smaller gauges for routine adult venipuncture unless absolutely necessary
- Be aware that even a 22G can cause hemolysis with high-volume draws if technique is rough
Cause 2: Excessive Suction During Tube Withdrawal
What happens: When you place a collection tube on the needle holder, a vacuum draws blood from the vein. If the vacuum is too strong (underfilled tube or needle holder issue), cells burst as they're pulled through the needle.
Prevention:
- Verify tube integrity: Check expiration dates and storage conditions. Expired tubes may have lost partial vacuum, creating compensatory suction.
- Verify needle holder: Needle holders with worn or damaged needle seats can create excessive vacuum. Replace if worn.
- Use butterfly slowly on high-vacuum draws: If drawing a serum separator tube (gold top) with a butterfly, which is slower, the extended draw time can increase hemolysis risk. Consider a straight needle for high-vacuum tubes when possible.
- Monitor tourniquet pressure: A tourniquet left on too long causes venous stasis and increases blood viscosity, which can create back-pressure hemolysis during tube transitions.
Cause 3: Rough Venipuncture Technique (Probe and Adjust)
What happens: Inserting the needle, missing the vein, and then probing around inside the tissue causes mechanical trauma to cells in and around the needle path.
Prevention:
- Palpate carefully before puncture. Locate the vein by feeling the vein path, diameter, and bounce-back (recoil).
- Insert at the correct angle: 15-30 degree angle (shallow, not deep). Too steep and you risk going through the back wall of the vein.
- Limit to one or two attempts. If you miss on the first or second try, stop and try a different site. Repeated probing guarantees hemolysis.
- Use a butterfly for difficult veins. The shorter needle, tubing buffer, and wing stabilization reduce the risk of probing trauma.
Cause 4: Vigorous Mixing of Collection Tubes
What happens: Shaking tubes hard after collection or inverting them too rapidly can rupture red blood cells, especially in serum separator tubes where centrifugal force hasn't yet separated cells.
Prevention:
- Mix gently. For tubes with additives (EDTA, SST, lithium heparin), invert 8-10 times using slow, smooth motions. Do not shake or vigorously agitate.
- Use proper mixing technique: Hold the tube horizontally and invert end-over-end. Do not flick the tube or hold it vertically and bounce it up and down.
- Follow tube-specific guidelines:
- Serum separator tubes (gold): 5 inversions only (tight additive mixing requirement is minimal)
- EDTA tubes (purple): 8-10 inversions (anticoagulant needs thorough mixing)
- Lithium heparin (green): 8-10 inversions
- Citrate (blue): 3-4 inversions (critical, over-mixing causes hemolysis)
- Don't squeeze tubes. Some phlebotomists instinctively squeeze specimens to check they are full. This forces additives into blood and ruptures cells.
Cause 5: Underfilling Tubes (Wrong Blood-to-Additive Ratio)
What happens: Serum separator tubes and coagulation tubes rely on precise blood-to-additive ratios. An underfilled tube has excess additive relative to blood, which desiccates cells and causes osmotic hemolysis.
Prevention:
- Fill each tube to the marked line. Tubes have fill lines for a reason. Respect them. An underfilled serum separator tube (gold) can cause dramatic hemolysis hours later during centrifugation.
- Know volume requirements by tube type:
- Most tubes: fill to line marked on tube (typically 5-10 mL)
- Coagulation tubes (blue): 2.7 mL or 3.2 mL depending on analyzer; underfilling skews PT/PTT results
- Pediatric/special populations: use smaller tubes with correspondingly smaller fill volumes
- Wait for adequate blood flow before inserting tubes. If blood flow is slow, wait until a steady drip is established; sudden tube insertion on a slow-flow vein can cause sputtering and aeration.
Cause 6: Slow Blood Flow and Extended Draw Time
What happens: A slow draw means blood spends more time in the needle and collection tube, increasing the time cells are exposed to foreign surfaces (needle metal, tube plastic, additives) and atmospheric oxygen. This increases hemolysis risk, especially with high-gauge needles (23G+).
Prevention:
- Ensure proper needle entry and vein access. If blood flow is slow, you may be in the vein partially or the needle bevel is against the vein wall. Adjust slightly.
- Release the tourniquet promptly. Keep the tourniquet on during collection (it helps maintain pressure), but remove it as soon as the first tube is filling adequately. Tourniquet time >1 minute increases hemolysis risk and falsely elevates potassium.
- Choose appropriate needle gauge for draw volume and vein quality. For a slow-flow vein, a 23G butterfly is acceptable because the shorter needle and tube buffer compensate for slow flow. For a high-volume draw on a slow vein, you may need to use two venipunctures rather than forcing a slow draw through a small needle.
- Avoid talking or distracting the patient during draw. Patient anxiety and muscle tension slow venous return. A calm patient = faster blood flow.
Cause 7: Improper Specimen Handling and Transport
What happens: Agitation during transport, exposure to extreme temperatures, or allowing specimens to sit unrefrigerated can cause hemolysis after collection.
Prevention:
- Transport carefully. Don't bounce specimens in a basket or drop them. Use a specimen carrier or tray designed to minimize jostling.
- Refrigerate time-sensitive specimens. Some tests (potassium, glucose, blood gases) degrade quickly at room temperature. Check your lab's transport protocol.
- Separate serum from cells promptly. Serum separator tubes should be centrifuged within 2 hours; the longer cells and serum sit together, the higher the risk of osmotic hemolysis.
- Don't leave specimens on top of warm equipment. Heat accelerates hemolysis.
Cause 8: Patient-Related Factors (Lipemia, Hemolytic Anemias, Polycythemia)
What happens: Some patients have inherent risk factors for hemolysis: hemolytic anemia (RBCs are already fragile), polycythemia (high RBC concentration increases hemolysis risk), lipemia (interferes with assays and can predispose to hemolysis), or fever/sepsis (altered RBC membrane).
Prevention:
- Know your patient's history. If a patient has a known hemolytic anemia or is on medications that affect RBCs (dapsone, sulfonamides), use extra care. Consider a butterfly needle and gentle handling even if the draw seems routine.
- Document pre-analytical factors. If a patient is febrile, critically ill, or in active hemolysis, note this when delivering the specimen. The lab may flag results or request a redraw with notation of the clinical context.
- Use good fasting protocol. Lipemia (high triglycerides) increases hemolysis risk. Ensure patients are properly fasted (12 hours overnight fasting for lipid panels) to minimize this confound.
Recognition: How to Spot Hemolyzed Specimens
Visual inspection is your first line of defense:
- Pale pink: Mild hemolysis
- Dark pink/red: Moderate to severe hemolysis
- Deep red/brown: Severe hemolysis (specimen will likely be rejected)
- Clear: No hemolysis
Some labs use automated hemolysis detection on their analyzers, but visual inspection during collection is your responsibility. If you collect a visibly hemolyzed specimen, note it immediately and communicate with the lab. Do not try to hide it or hope the automated system misses it.
What to Do When Hemolysis Happens Anyway
Sometimes, despite best efforts, hemolysis occurs. Here's the protocol:
- Notify the lab immediately. Do not send a hemolyzed specimen unless the lab explicitly approves it (rare, only for specific tests like hemoglobin where hemolysis may be irrelevant).
- Recollect. Draw a new specimen from a different site if possible. If you suspect your technique caused the hemolysis, adjust: use a larger gauge, slower insertion, gentler mixing.
- Communicate with the patient. Explain that you need a redraw and be honest about why. Most patients are understanding, especially if you explain that you're ensuring accuracy of their results.
- Document the incident. Record the first draw (time, site, gauge) and the reason for recollection. This protects you and provides data for quality improvement.
ASCP Exam Focus: Hemolysis Prevention
The ASCP PBT exam frequently tests hemolysis prevention. Typical questions:
Example: "A potassium result comes back at 6.8 mEq/L, which is elevated. The patient is asymptomatic. What is the phlebotomist's responsibility?"
Answer: Recognize that elevated potassium from a single draw may be a false positive due to hemolysis (from improper technique, underfilled tube, or extended tourniquet time). Communicate with the lab about the possibility of hemolysis and request a redraw if the specimen appeared hemolyzed or if collection technique was questionable.
Know the causes, know how to prevent them, and you'll master this exam topic and real-world practice.
Summary: Prevention Is Everything
Hemolysis is preventable. Every hemolyzed specimen is a quality failure and a patient safety issue, it may delay diagnosis or lead to unnecessary treatment. Your technique, your attention to detail, and your understanding of the causes will determine whether your lab has a hemolysis rate of 0.5% (excellent) or 5% (poor). Be the phlebotomist who prevents hemolysis.