5 Reasons Why Your Blood Sample May Be Rejected in the Lab

 

Have you ever had your blood drawn, only to be told that you need to have it collected again?

It can be frustrating to hear that your blood sample needs to be recollected, especially when you already went through the process of getting your blood drawn.

But there are actually several reasons why a laboratory may reject a specimen. As a Medical Laboratory Technologist (MLT) working in Hematology, these are some of the common reasons I see.

1. NSQ  <Not Sufficient (or insufficient) Quantity>

One of the simplest reasons a specimen may be rejected is that there isn't enough blood in the tube.

Every test requires a certain amount of specimen. If there isn't enough, the laboratory may not be able to perform the test accurately.

For some tests, the amount of blood in the tube is especially important because the blood-to-anticoagulant ratio needs to be correct to produce a reliable result.

For example, coagulation tubes MUST be filled to the line indicated on the tube. 

So even though there may be "some blood" in the tube, it doesn't necessarily mean there is enough to run the test.

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2. Hemolyzed

Hemolysis happens when red blood cells break apart and release their contents into the plasma or serum.

You may sometimes hear the laboratory describe a sample as "hemolyzed."

A small amount of hemolysis may not affect every test, but significant hemolysis can interfere with certain laboratory results.

Why does this happen?

There are many possible reasons. For example, the blood draw itself can sometimes cause hemolysis if the collection process is difficult or the specimen is handled improperly.

When the hemolysis is significant enough to potentially affect the accuracy of the result, the laboratory may reject the specimen and request a new collection.

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3. Clotted

If a blood sample is collected in a tube containing an anticoagulant and mixed after the draw, the blood should NOT clot.

So what happens if we find a clot? Most of the time it will be rejected and asked for a re-draw. However, for irretrievable specimen such as bone marrow or some types of body fluid, it may still be processed according to the laboratory protocol.

This is particularly important for tests such as CBC, where we need the blood cells to be evenly suspended throughout the specimen.

Imagine trying to count all the cells in a tube when some of them are stuck together in a clot. The analyzer will not be able to produce an accurate result.

This is one reason why gently mixing the tube after collection is so important.

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4. Too Old

Blood samples don't last forever.

Different laboratory tests have different specimen stability requirements, meaning there is a specific amount of time during which the specimen can be reliably tested.

If a specimen arrives at the laboratory too late, the laboratory may not be able to guarantee that the result is accurate.

This can be especially important for certain tests where blood cells or other components of the specimen change over time.

So sometimes the problem isn't the quality of the blood when it was collected — it's simply that too much time has passed.

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5. Wrong Collection Tube

This is another common reason a specimen may be rejected.

Different laboratory tests require different types of collection tubes because the tubes contain different anticoagulants, additives, or preservatives.

For example:

• Lavender/purple top → EDTA → commonly used for hematology tests such as CBC
• Light blue top → Sodium citrate → commonly used for coagulation testing
• Green top → Heparin → commonly used for chemistry tests
• Red or gold top → Serum specimens → commonly used for various chemistry and serology tests

Using the wrong tube can affect the specimen and potentially produce an inaccurate result.

That's why the tube isn't just a container for the blood — the tube itself is part of the test.

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Why Does Specimen Rejection Matter?

It might seem inconvenient when the laboratory asks for another blood sample, but there is a reason behind it.

The goal isn't simply to get a random number as a result.

The most important thing is to product a reliable result that accurately reflects the patient's condition.

If the specimen is insufficient, hemolyzed, clotted, too old, or collected in the wrong tube, reporting a result could potentially be misleading and lead to the doctor making an incorrect clinical decision.

So when a laboratory rejects a specimen, it isn't because the laboratory "doesn't want to run the test."

Sometimes, rejecting the specimen is the safer choice.

And yes, it means someone may have to draw another tube of blood; it is definitely inconvenient for both the patient and the phlebotomist. But we are not rejecting without a good reason.

Garbage in, Garbage out.