Specimen Rejection Criteria: Why Labs Reject Blood Samples

You drew the blood. You labeled the tube. You sent it to the lab. Then you get a call: specimen rejected. What went wrong?

Specimen rejection is one of the most frustrating parts of laboratory work, and it costs real time — a rejected specimen means the patient gets stuck again, results are delayed, and clinical decisions get pushed back. Knowing why labs reject specimens is not just useful test knowledge. It is the foundation of doing your job right the first time.

This article covers every major rejection reason you will see on the NHA CPT exam and in clinical practice: what causes each one, how to prevent it, and what happens if a compromised specimen gets tested anyway.

Why Specimen Quality Matters

The lab can only work with what you give it. Once a tube leaves your hands, the analyzer cannot distinguish a good specimen from a compromised one — it will run the sample and produce a number. That number might be wrong. And a wrong number can lead to a wrong diagnosis, a wrong medication dose, or a missed critical value.

This is why labs have rejection policies. A result that cannot be trusted is worse than no result at all.

The 8 Most Common Rejection Reasons

1. Hemolysis

Hemolysis is the number one cause of specimen rejection, accounting for 40–70% of all rejected specimens depending on the institution. It happens when red blood cells rupture and release their intracellular contents — primarily hemoglobin — into the serum or plasma.

What causes it:

• Using too small a needle (23g or smaller on a large vein creates turbulence)
• Pulling back too hard on a syringe plunger
• Mixing tubes too vigorously — flip-invert, do not shake
• Transferring blood from a syringe to a tube too forcefully
• Drawing from a site that is still wet with alcohol (always let it dry fully)
• Prolonged tourniquet time (over 1 minute)
• Drawing from a hematoma or bruised area
• Exposing the specimen to extreme temperatures

How to prevent it: Use the correct needle gauge for the vein. Allow alcohol to dry completely before puncture. Mix tubes gently with 8–10 slow inversions. Avoid drawing from areas with obvious tissue damage.

What happens if a hemolyzed specimen is tested anyway: Hemolysis artificially elevates potassium (K+) — sometimes dramatically, because red cells contain 20–30 times more potassium than plasma. A patient with a real potassium of 4.0 mEq/L could show 7.0 or higher on a hemolyzed sample, triggering cardiac emergency protocols unnecessarily. It also falsely elevates LDH, AST, ALT, magnesium, phosphorus, and ammonia. Hemoglobin released into the sample interferes with colorimetric and spectrophotometric assays, producing inaccurate results across a wide range of tests.

2. Clotted Specimen in an Anticoagulant Tube

Tubes containing anticoagulants — EDTA (lavender), sodium citrate (light blue), heparin (green) — require adequate mixing to prevent coagulation. A clot in one of these tubes means the anticoagulant failed to bind the calcium or activate its mechanism.

What causes it:

• Not inverting the tube the required number of times immediately after collection
• Waiting too long before mixing
• Insufficient blood volume relative to additive (underfilling compounds the problem)
• Slow or difficult draw that allows partial clotting during collection

How to prevent it: Mix immediately after capping — do not set the tube down first. Use the correct number of inversions: EDTA requires 8–10, sodium citrate requires 3–4, heparin requires 8–10. If the draw is slow, mix the tube partway through collection when possible.

What happens if tested anyway: For a CBC, clots consume platelets and fibrin, producing falsely low platelet counts and inaccurate WBC differentials. Clots in a coagulation tube (PT/INR, PTT) invalidate results entirely because the clotting factors have already been activated and consumed. Reporting a PTT from a clotted light blue tube could suggest a patient is adequately anticoagulated when they are not.

3. Lipemia

A lipemic specimen appears milky or cloudy due to excess triglycerides or chylomicrons in the blood. While this is a patient condition, not a collection error, it is still a rejection reason when lipid levels exceed the lab's interference threshold.

What causes it:

• Drawing from a patient who did not fast when fasting was required
• Drawing within 4–6 hours of a high-fat meal
• Patients with hypertriglyceridemia may produce lipemic samples even while fasting

How to prevent it: Confirm fasting status before drawing lipid panels and any test that requires a fasting specimen. Document non-fasting status and notify the ordering provider. Some labs will accept non-fasting specimens with a notation; others reject outright.

What happens if tested anyway: Lipids scatter light and absorb at wavelengths used by spectrophotometric analyzers, interfering with bilirubin, total protein, albumin, LDH, and many chemistry tests. Lipemia also physically alters the aqueous phase of plasma, artificially lowering sodium and other electrolytes through electrolyte exclusion error. Results from a severely lipemic sample can be wildly inaccurate and clinically misleading.

4. Insufficient Volume (Short Draw / QNS)

QNS stands for "quantity not sufficient." Each tube is engineered with a specific blood-to-additive ratio. When a tube is underfilled, that ratio is thrown off.

What causes it:

• Collapsed vein during draw
• Tube vacuum exhausted or partially lost (expired or damaged tube)
• Patient movement
• Pulling the needle out before the tube filled
• Small, fragile, or rolling veins that could not sustain the draw

How to prevent it: Watch the tube fill and wait for blood flow to stop naturally before removing. For difficult draws, use a smaller tube or a butterfly set to manage vacuum. Never try to refill a partially filled tube — it has already broken the sterile barrier.

What happens if tested anyway: In a light blue coagulation tube, the correct blood-to-citrate ratio is exactly 9:1. A short draw means too much citrate relative to plasma, which chelates excess calcium and over-anticoagulates the sample, producing artificially prolonged PT and PTT. This is one of the most dangerous short-draw scenarios — a patient might appear to need less anticoagulant when they actually need more. For CBC tubes, low volume can produce unreliable cell counts.

5. Wrong Tube Type

Every test is designed to run on a specific tube additive. Collecting in the wrong tube is a pre-analytical error that cannot be corrected after the fact.

What causes it:

• Not checking the test order against the required tube color before drawing
• Rushing through a multi-tube draw and grabbing the wrong tube
• Misreading tube color (red vs. gold/SST, light blue vs. lavender in poor lighting)

How to prevent it: Verify the test order before selecting tubes. Know your order of draw: Yellow/SPS → Light Blue → Red/Gold SST → Green → Lavender → Gray. If you are uncertain which tube a test requires, look it up before drawing.

What happens if tested anyway: Running a PT/INR from an EDTA tube instead of a light blue tube produces meaningless results — EDTA chelates calcium completely and disrupts the coagulation cascade in a way that cannot be corrected mathematically. Running a glucose in a red-top tube instead of a gray-top (sodium fluoride) allows glycolysis to continue, lowering glucose approximately 10 mg/dL per hour at room temperature. A patient with actual hypoglycemia could look normal; a normal patient could look hypoglycemic.

6. Mislabeled or Unlabeled Specimen

This is one of the most consequential rejection reasons because it is a patient safety issue, not just a quality issue. A label connects a tube to a patient. Without a correct label, results cannot be attributed to the right person.

What causes it:

• Printing labels in advance and applying them at the wrong time or to the wrong tube
• Writing patient information by hand and making errors
• Not labeling at bedside — labeling at the nurses' station "just this once"
• Using a label from the wrong patient's order set

How to prevent it: Label every tube at the bedside, immediately after collection, before you leave the patient. Use two patient identifiers every time: full name and date of birth are the standard. Scan the patient wristband if barcode technology is available. Never prelabel tubes.

What happens if tested anyway: If a mislabeled specimen is processed, results are attributed to the wrong patient. A blood type and crossmatch performed on the wrong patient's blood leads to transfusion of incompatible blood — this is one of the leading causes of transfusion-related fatalities. Joint Commission and CAP accreditation standards require rejection of any specimen that cannot be definitively linked to the correct patient.

7. Expired Tube

Vacuum tubes have expiration dates printed on the label. After that date, the vacuum may be compromised and additives may have degraded.

What causes it:

• Using old stock without checking dates
• Drawing from a tube that was stored improperly (excess heat or humidity accelerates expiration)
• High-volume environments where tubes sit in drawers for extended periods

How to prevent it: Check the expiration date before every draw — this takes two seconds and prevents a recollection. Rotate stock, using older tubes first. Store tubes at room temperature away from heat and moisture.

What happens if tested anyway: An expired tube may not fill completely because the vacuum is weak, producing an underfilled specimen with all the problems described above. In additive tubes, anticoagulants and preservatives degrade over time. An expired gray-top (sodium fluoride) may not inhibit glycolysis adequately, allowing glucose to fall before the sample reaches the analyzer. Expired coagulation tubes can produce erratic PT/PTT results.

8. Contaminated Specimen

Contamination can come from IV fluids, other patients' specimens, environmental sources, or cross-contamination between tubes.

What causes it:

• Drawing from the arm ipsilateral to a running IV without adequate distal draw technique or a documented hold
• Drawing from a central line or PICC line without proper discard volume
• Improper skin prep allowing bacteria or antiseptic to enter the specimen
• Cross-contamination from an additive tube backflow into a preceding tube (reason for the order of draw)

How to prevent it: When drawing above or near an IV site, turn off the infusion for a minimum of 2 minutes and draw distal to the IV. For line draws, discard the first 5–10 mL (or per institutional policy). Follow the correct order of draw to prevent additive carryover. Use proper aseptic technique for blood cultures (70% isopropyl alcohol followed by povidone-iodine or chlorhexidine).

What happens if tested anyway: IV fluid contamination dilutes the specimen and introduces whatever is in the IV — dextrose infusion will elevate glucose, saline will dilute electrolytes. A patient on a potassium drip drawn from the ipsilateral arm will show a falsely elevated potassium. Blood cultures contaminated during collection grow skin flora (Staph epidermidis, Cutibacterium), leading to unnecessary antibiotic treatment and prolonged hospitalizations.

Summary: Quick Reference Table