Hemolysis in Phlebotomy: Causes, Prevention, and the Lab Tests It Affects

Hemolysis is the rupture of red blood cells. When the cell membrane breaks, hemoglobin and everything else inside the cell leaks out into the surrounding plasma or serum. After the tube is centrifuged, you can see it. The plasma or serum looks pink, red, or in bad cases almost the color of cherry juice. A clean specimen looks straw yellow. Once a tube goes pink, the lab has a problem.

Hemolysis is the single most common reason clinical labs reject blood specimens. Studies of pre-analytical errors put it at the top of the list across hospital and outpatient settings. That matters for two reasons. The first is patient care. A hemolyzed potassium result can look like a cardiac emergency that does not exist. The second is workflow. Every rejection means a recollection, a delay, a frustrated patient, and a phone call from the lab. The good news is that the vast majority of hemolysis a phlebotomist sees is preventable, because it happens during or immediately after the draw.

In Vitro vs In Vivo Hemolysis

There are two ways red cells can break. The first is in vitro hemolysis, which means the cells were intact in the patient but ruptured outside the body, in the tube, during or after collection. This is the kind a phlebotomist causes and the kind a phlebotomist can prevent. It accounts for the overwhelming majority of hemolyzed specimens labs see.

The second is in vivo hemolysis, which means red cells are being destroyed inside the patient. This is a real clinical condition. Causes include hemolytic anemia, transfusion reactions, autoimmune disease, certain infections like malaria, mechanical heart valves, and severe burns. When in vivo hemolysis is happening, the patient will usually have other lab signs: low haptoglobin, elevated indirect bilirubin, elevated reticulocyte count, and sometimes hemoglobinuria. The specimen still looks hemolyzed, but the cause is the patient, not the draw.

How does the lab tell them apart? They cannot always. If a specimen shows up hemolyzed and the phlebotomist swears the draw was clean, the lab will sometimes ask for a redraw to confirm. If the second specimen is also hemolyzed, in vivo is more likely. If the second one is clean, the first was a draw issue. The point for a phlebotomist is simple: do everything you can to make sure the hemolysis is not your fault, because the patient cannot tell you whether their cells are fragile until the lab has already worked the sample.

Top Causes a Phlebotomist Can Prevent

Almost every in vitro hemolysis case traces back to mechanical or chemical stress on the red cells during collection. Here are the most common culprits, in roughly the order they show up in the field.

1. Needle gauge too small for the draw. A 25 gauge needle has a much narrower bore than a 21 or 22. When you pull a full vacuum tube of blood through a 25, the cells get squeezed and sheared as they pass through. For routine adult venipuncture, 21 or 22 gauge is standard. Save the 23 gauge butterfly for difficult or pediatric draws. A 25 gauge is generally too small for routine evacuated tube collection.
2. Vigorous shaking instead of gentle inversion. Tubes with additives need mixing, but mixing means slow, complete inversions. Five for clot activator tubes, eight to ten for EDTA, heparin, and citrate. Shaking the tube like a cocktail will hemolyze the sample and create micro-clots at the same time.
3. Forcing blood from a syringe through a needle into a tube. Pushing the syringe plunger to force blood through a small needle into a tube creates exactly the kind of shear stress that breaks cells. This is why a blood transfer device exists. The transfer device lets the tube vacuum pull blood at its own pace. Never push.
4. Drawing through a hematoma. Pooled blood under the skin is already partially clotted and damaged. Drawing through it pulls in red cells that are already compromised. If you see a hematoma forming during the draw or starting from a previous stick, choose a different site.
5. Prolonged tourniquet time. Over one minute and you start to get hemoconcentration and stress on the cells. The longer the tourniquet stays on, the more likely you are to hemolyze the sample and to skew tests like potassium, calcium, and lactate. If you cannot find the vein in a minute, release the tourniquet, let the arm rest, and reapply.
6. Probing for a vein. Once the needle is in, fishing around with the tip damages tissue and causes bleeding under the skin. That blood gets pulled into the tube along with the venous blood you want. Probing also tends to extend the draw, which compounds the tourniquet problem. If the first stick misses, withdraw and try again at a different site.
7. Butterfly with a partial fill on a coagulation tube. A butterfly set has dead space air in the tubing. If your only tube is a light blue, the air will displace blood and underfill the tube. Underfill not only ruins the citrate ratio for PT/aPTT, it also tends to produce hemolysis because the tube draws blood at irregular pressure. The fix is the discard tube. Draw a small clear tube first to flush the air.
8. Pneumatic tube transport with tubes loose in the carrier. Pneumatic systems shake samples around. If tubes are not secured in foam inserts, they bounce, and bouncing hemolyzes. Some hospitals limit which specimens can travel by tube for this reason. Know your facility policy.
9. Frothing from a poor needle-to-tube seal. If the needle is not fully engaged with the tube stopper, air leaks in along with the blood. The mix of air and blood foams. Foam means hemolysis. Push the tube fully onto the back needle and listen for the vacuum to engage.
10. Mixing too hard. Once the tube is filled, invert it. Do not flick it, shake it, or roll it between your palms. Slow inversion mixes additive without breaking cells.

None of these are exotic. They show up every shift somewhere. The phlebotomist who avoids hemolysis is the one who treats every step as deliberate.

Less Common In Vitro Causes

A few causes show up less often but are worth knowing about because they can ambush a careful phlebotomist.

Extreme temperature. Specimens left in a hot car or stored next to a freezer pack without insulation can hemolyze. Both heat and freezing damage the red cell membrane. Whole blood should not be frozen. If a specimen needs to be cold, it goes on wet ice or in a refrigerator, never in a freezer.

Contamination with water or alcohol. Water in a tube from a poorly dried venipuncture site, or worse, residual alcohol from skin prep that was not allowed to dry, can lyse red cells. Alcohol also stings the patient on insertion, which is a separate quality issue. Let the alcohol air dry. Do not blow on it, do not wipe it off.

Prolonged storage of whole blood. Blood left sitting as whole blood, not centrifuged and not separated, becomes more fragile over time. Some tests have stability requirements that demand serum or plasma be separated within a set window, often two hours. The lab cares about this. If you are responsible for handing off specimens, get them moving.

Tests Most Affected by Hemolysis

When red cells rupture, their contents spill into the plasma or serum. Some of those contents are tested directly, which means hemolysis dramatically falsifies the result. Others are not tested or are barely affected.

The tests most affected:

• Potassium (K+). Red cells contain about 25 times more potassium than plasma. Even mild hemolysis can falsely elevate the reported potassium. A patient with a true potassium of 4.0 mEq/L can show 6.5 in a hemolyzed sample. That looks like hyperkalemia, which can trigger an unnecessary cardiac workup.
• LDH (lactate dehydrogenase). LDH is heavily concentrated inside red cells. Hemolysis falsely elevates LDH by a large margin and is the most common reason for an unexpectedly high LDH.
• AST (aspartate aminotransferase). Also concentrated in red cells. Hemolysis bumps it up.
• Magnesium. Falsely elevated.
• Phosphorus. Falsely elevated.
• Ammonia. Falsely elevated, and ammonia is already a fragile analyte for other reasons.
• Iron. Falsely elevated, which can mask iron deficiency or fake iron overload.

Tests less affected by hemolysis include glucose and BUN. Sodium can actually appear slightly low in heavily hemolyzed samples because the lysed red cell water dilutes the plasma. Hemoglobin and hematocrit on a CBC are usually still reportable because the analyzer measures the cells differently, but the lab will still flag the specimen.

The practical takeaway is that a hemolyzed potassium is a clinical emergency in waiting if no one catches it. The lab will usually flag a hemolyzed sample with an HI (hemolysis index) and may refuse to report potassium altogether. That is the right call. A redraw is far safer than a wrong number.

Prevention Checklist

Run through this before and during every draw.

• Choose a 21 or 22 gauge needle for routine adult venipuncture. Use a 23 gauge butterfly only when needed.
• Let the alcohol air dry fully before inserting the needle.
• Keep the tourniquet on for less than one minute. If you cannot palpate the vein, release and try again.
• Avoid drawing through a hematoma. Pick a different site or arm.
• Anchor the vein, insert smoothly, and do not probe.
• Engage the tube fully so the vacuum draws steadily without air leak.
• For a butterfly draw with a coagulation tube, draw a discard tube first to clear the dead space.
• Fill the tube to the indicated line. Underfilled tubes are more likely to hemolyze and to have wrong additive ratios.
• Use a blood transfer device for syringe collections. Never push the plunger.
• Invert tubes gently and the correct number of times for the additive.
• Secure tubes in foam inserts before sending through pneumatic tube systems.
• Keep specimens at the right temperature. No hot cars, no freezers for whole blood.
• Move specimens to the lab promptly so they can be processed within stability windows.

What to Do When You See Hemolysis

Sometimes you spin a specimen, you see pink plasma, and the cause is obvious. Other times the lab calls you. Either way, document what you see. Most labs grade hemolysis on a visual scale.

Follow your lab protocol on grading and reporting. If the lab requests a recollection, do it as soon as possible and note in the system that the original was hemolyzed. Do not argue with the lab tech about whether the level was acceptable. They are using validated criteria.

If the same patient hemolyzes a second time with a clean draw, the next step is usually a conversation between the ordering provider and the lab to consider in vivo causes. That is not a phlebotomist call to make, but you should flag it.

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