Hemolysis in Blood Draws: What Causes It and How to Prevent It
By the OQMPS team ·
Hemolysis is red blood cells breaking open and spilling what’s inside them into the sample. A little of it can make a potassium result look dangerously high or hide a real problem. Almost all hemolysis happens during or after the draw, not in the patient’s body, so most of it is preventable at collection.
Quick answer. The main causes during a draw: a needle that doesn’t suit the vein, wet alcohol, fist pumping, probing for the vein, too much vacuum on a small vein, forcing blood from a syringe into a tube, underfilled tubes, shaking instead of inverting, and squeezing a fingerstick. Each has a simple fix, listed below.
In this guide
What hemolysis is, and why it matters
What causes hemolysis during a blood draw
What hemolysis is, and why it matters
Red blood cells hold far more of some substances than the liquid around them. When they burst, those substances leak into the serum or plasma the lab tests. Serum looks pink or red once free hemoglobin passes about 0.3 g/L, but several results change before you can see anything.
It is the most common reason a lab rejects a sample. A review for the CDC’s Laboratory Medicine Best Practices program found hemolysis in up to 3.3% of routine samples and behind 40% to 70% of all unsuitable ones. A benchmark often cited in the literature is 2% or lower. Emergency departments run higher: often 7% to 20%, and over 30% in some studies. In a College of American Pathologists study of cancelled tests, hemolysis was the top reason at the collection stage.
Very little of it starts in the body. In one study, 3.3% of samples sent to the lab were hemolyzed, and only 3.2% of those reflected hemolysis inside the patient. The rest happened in the tube.
Which tests it throws off
| Test | What happens | Why |
|---|---|---|
| Potassium | Can read falsely high | Red cells hold about 25 times more potassium than plasma. Hemolysis can raise it enough to look like a medical emergency |
| AST (liver enzyme) | Can read falsely high | Red cells hold about 40 times more |
| LD (lactate dehydrogenase) | Can read falsely high | The most sensitive test; it shifts before hemolysis is visible |
| Bilirubin | Can read falsely low | Hemoglobin interferes with some test methods |
| Clotting tests, blood typing | Sample rejected | Most labs won’t run them on a hemolyzed sample |
In all, hemolysis can bias 39 different lab tests.
What causes hemolysis during a blood draw
| Cause | What to do instead |
|---|---|
| Alcohol still wet at the stick | Scrub for 30 seconds and let it air-dry for 30 more. Don’t blow on it or wipe it |
| Fist pumping | Ask for a closed fist, held still, and have the patient relax it once blood flows. In one study, pumping raised potassium about 13% |
| Probing for the vein | Line up before you go in. Don’t dig. If the needle is only partly in the vein, blood can froth into the tube |
| Too much vacuum on a small vein | Use a smaller tube or a smaller syringe. WHO warns against 10 to 20 mL syringes on small veins |
| Forcing blood from a syringe into a tube | Use a transfer device and let the tube’s vacuum pull the blood in. Never push the plunger |
| Pulling a syringe plunger too hard | Draw back slowly and steadily |
| Drawing through an IV catheter | Use a fresh straight-needle stick. Across 11 studies, straight-needle draws cut hemolysis by about 84% compared with drawing from an IV start |
| Underfilled tubes | Fill to the line. Underfilling raises the share of additive, and in a light blue tube it also throws off clotting results |
| Shaking the tubes | Invert gently the number of times the tube calls for. See the inversion counts in our order of draw chart |
| Squeezing a fingerstick or heelstick | Warm the site first, wipe away the first drop, and let the blood flow without milking. WHO says heavy squeezing causes hemolysis |
| Drawing from a hematoma | Choose another site. See the best veins for blood draws |
| Tourniquet on too long | One minute at most. This is a standard rule, though the research tying it to hemolysis is thin |
What about needle size? A needle that’s too thin for the vein can damage cells, and one that’s too big can cause turbulence or a hematoma. For adults, WHO’s guidelines list 21, 22 and 23 gauge needles, and 21 gauge is a common choice for a routine draw. For small veins, older patients and children, WHO lists a 22-gauge needle or a 23-gauge winged set (a butterfly), and its section on children warns that 25 gauge and smaller may raise the risk of hemolysis. A 2005 study found no difference in hemolysis between a butterfly and a straight needle of the same 21 gauge. So match the size to the vein. Our butterfly needle guide has the details.
Why emergency departments see more. Much of the hemolysis in EDs comes from drawing through new IV lines to save the patient a second stick. Phlebotomists rarely draw from lines, but it explains why a fresh stick is better. In one randomized study, butterfly draws by ED nurses were hemolyzed less than 1% of the time, against 5.6% for IV draws. One Boston hospital that required separate straight-needle draws saw its rate fall from 23% to 6.6%.
After the draw
Hemolysis can still happen in rough handling and transport. Pneumatic tube systems are mixed. In one study, a fast system raised potassium and LD in tubes that hadn’t been spun yet; spun samples were fine. An ED study found no difference from hand-carried samples, though only 92 samples were hand-carried. Each hospital is supposed to check its own system.
What happens to a hemolyzed sample
Analyzers measure a hemolysis index, because eyeballing it is unreliable, even for trained lab staff. Above the lab’s cutoff for a given test, that result is withheld or flagged and the sample usually has to be redrawn. That means another stick for the patient and a delay. In one ED study, visits with a hemolyzed potassium sample ran about an hour longer on average (half an hour at the median), though other factors may explain part of that.
If your lab report says “hemolyzed.” It almost always means the sample was damaged, not that something is wrong with your blood. Only about 3 in 100 hemolyzed samples reflect hemolysis in the body. The lab will usually ask for a redraw. If your doctor has other reasons to suspect a blood problem, they’ll order tests for that.
A hemolysis checklist
- Choose a good vein, and a needle that suits it.
- A closed fist, no pumping.
- Tourniquet off within a minute.
- Alcohol fully dry before the stick.
- No probing.
- Tubes filled to the line, in the order of draw.
- Gentle inversions, never shaking.
- A transfer device for syringe draws; never push the plunger.
- No squeezing on fingersticks.
Practice the habits that prevent it
Good collection technique is most of hemolysis prevention, and it is what OQMPS phlebotomy classes drill: 3 days in class plus self-paced home study, in Arizona, Oregon, Texas, Utah and Washington. Take the course online if you can’t get to a campus, or try our free phlebotomy practice test.
Frequently asked questions
What causes hemolysis in a blood draw?
Mostly technique: wet alcohol, fist pumping, probing, too much vacuum on a small vein, forcing blood through a needle from a syringe, underfilled tubes, shaking tubes and squeezing fingersticks.
What does "hemolyzed" mean on a lab report?
That red cells in the sample broke open, almost always during or after collection. It usually means a redraw, not a problem with your blood.
Can a hemolyzed blood sample still be tested?
Some tests can still run if the hemolysis is mild. Others, including potassium and clotting tests, are usually withheld and the sample is redrawn. Each lab sets its own cutoffs.
Why does hemolysis raise potassium?
Red blood cells hold about 25 times more potassium than plasma. When they burst, that potassium leaks into the sample and the result reads falsely high.
Does a small needle cause hemolysis?
Very thin needles can. WHO's guidelines list 21 to 23 gauge needles for adults and a 23-gauge butterfly for small veins and children, and their section on children warns that 25 gauge and smaller may raise the risk of hemolysis.
Does shaking a blood tube cause hemolysis?
Yes. Tubes should be inverted gently the recommended number of times, never shaken.
How can you tell if a sample is hemolyzed?
Hemolyzed serum or plasma looks pink to red after spinning. But looking is unreliable, so labs measure a hemolysis index on the analyzer.
Sources
- WHO guidelines on drawing blood, 2010 (CDC Stacks copy)
- Heyer et al. 2012, Clin Biochem, CDC LMBP review (also the source for Lowe 2008 and Straszewski 2011)
- Thomas 2002, EJIFCC
- Koseoglu et al. 2011, Biochemia Medica
- Simundic et al. 2010, Biochemia Medica
- Lippi et al. 2013, Biochemia Medica: IV catheters and hemolysis
- Lippi et al. 2005, Clin Chem Lab Med: butterfly vs straight needle
- Acute Care Testing: “Make a fist, please”
- CAP TODAY: Q-Probes test cancellation study
- Evliyaoğlu et al. 2012, J Clin Lab Anal
- Phelan et al. 2016, West J Emerg Med
- Phelan et al. 2020, J Appl Lab Med
- Akron Children's Hospital: Performing a Venipuncture