Skip to content
SearchProgress
Display

Display

Theme
Density
Text size

Sign in

Add your earlier progress to your account?

Study progress is waiting to be saved

Is the specimen fit to test?

15 min

  • Decide whether a clot, short fill, wrong tube, or delay makes a specimen unsuitable
  • Recognize high potassium with low calcium as a sign of EDTA carryover
  • Recognize when fluid from an infusion line has diluted or spiked the specimen

Read the full reference

Try first

Try first

A sodium citrate tube for a prothrombin time (PT) arrives filled to about half of its fill line. It is labeled correctly and has no clot. What do you do?

Right. The next section explains why.

The next section explains it.

The next section explains it.

The next section explains it.

Get the idea

Reasons to reject

Before a specimen is tested, someone decides whether it can give a true result for the test ordered. Each laboratory writes acceptance criteria, and each criterion ties a defect to what it would do to the result.1 The common reasons to reject are:1

  • an unlabeled tube, or one whose label and request disagree
  • a leaking container
  • the wrong tube or additive
  • a clot in an anticoagulated tube
  • too little specimen for the additive or the test
  • a delay or temperature outside the test's stability limit

Citrate tubes show why fill matters. Coagulation testing depends on nine parts blood to one part citrate, and a short draw leaves extra citrate that prolongs clotting times.2 Hemolysis, icterus and lipemia are judged test by test against each method's own interference limits.3

Two problems leave no sign on the tube and show up in the results.

EDTA carryover

  • Potassium rises and calcium falls, often both far past anything the patient's history makes likely.
  • The cause is potassium EDTA from a hematology tube, which adds potassium and binds calcium.
  • It happens when blood from a syringe or open system reaches an EDTA tube before a serum or heparin tube, or when blood is poured from one tube into another.4

Infusion contamination

Blood drawn from a line or above an infusion site can carry the infusion fluid into the tube.

  • With normal saline, chloride climbs, sodium barely moves, and potassium, calcium, urea nitrogen and creatinine all fall by about the same fraction.
  • The saline causes that pattern. It pulls sodium and chloride toward its own 154 mmol/L and dilutes everything saline lacks.
  • Fluids containing dextrose drive glucose very high.5

What to do in either case

The tube itself is the problem, so rerunning it only reproduces the same wrong numbers and makes them look confirmed.

  1. Do not release the affected results.
  2. Recollect the specimen correctly, by venipuncture away from the infusion or in the right order of draw.4,5

Acceptable quality control does not help here either. Controls check the analyzer, and the analyzer measured this tube accurately.1

References
  1. World Health Organization. Laboratory Quality Management System: Handbook. World Health Organization; 2011. Accessed September 23, 2026.
  2. Clinical and Laboratory Standards Institute. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays. 6th ed. CLSI standard H21. Clinical and Laboratory Standards Institute; 2024. Accessed September 23, 2026. https://clsi.org/shop/standards/h21/
  3. Clinical and Laboratory Standards Institute. Interference Testing in Clinical Chemistry. 3rd ed. CLSI guideline EP07 Plus. Clinical and Laboratory Standards Institute; 2018. Reaffirmed October 2022. Accessed September 23, 2026. https://clsi.org/shop/standards/ep07-plus/
  4. Lorde N, Mahapatra S, Kalaria T, Gama R. Potassium ethylenediaminetetraacetic acid (kEDTA) sample cross-contamination: prevalence, consequences, identification, mechanisms, prevention and mitigation. J Lab Precis Med. 2024;9:34. doi:10.21037/jlpm-24-42
  5. Shean RC, Maucione C, Spies NC. Intravenous fluid-induced specimen contamination and detection strategies in clinical laboratories: a narrative review. J Lab Precis Med. 2025;10:23. doi:10.21037/jlpm-25-29

Watch one

A basic metabolic panel from a medical-surgical floor shows potassium 8.1 mmol/L and calcium 4.6 mg/dL.

  • Yesterday this patient's potassium was 4.0 mmol/L and calcium 9.2 mg/dL.
  • The hemolysis index is not raised.
  • The collection note says the blood was drawn by syringe and divided among a lavender EDTA tube and a green heparin tube, in that order.

What do you do with the panel?

  1. Check the indices: hemolysis is not raised, so broken red cells do not explain the potassium.

    Hemolysis is the most common cause of a falsely high potassium, and the index rules it in or out at once.

  2. Compare with yesterday: potassium doubled and calcium halved in one day.

    A change this large overnight is more often the specimen than the patient.

  3. Read the pair: a high potassium with a very low calcium is the pattern of EDTA carryover.

    Potassium EDTA adds potassium and binds calcium, so the two move in opposite directions together.

  4. Check the collection: the syringe filled the EDTA tube first and then the heparin tube.

    The collection record shows whether EDTA could have reached the heparin tube.

  5. Do not release the potassium or calcium, and request a new specimen collected in the correct order of draw.

    A repeat on the same tube would measure the same contamination.

Treat the panel as EDTA carryover: do not release the potassium or calcium, and request a redraw.

Your turn

Problem 1 of 3

A serum chemistry panel shows potassium 7.9 mmol/L and calcium 4.1 mg/dL. The same patient's results were within their intervals the day before. Which collection problem best fits this pair?

Correct. K2EDTA adds potassium and chelates calcium, so potassium rises and calcium falls. A specimen drawn out of order, or poured from an EDTA tube, is unsuitable. Request a new collection.

Incorrect. Potassium leaks from cells when separation is delayed, but the delay gives no reason for the very low calcium.

Incorrect. Citrate binds calcium, but it is a sodium salt and adds no potassium, so it cannot explain the high potassium.

Hint
  1. Look at the two results together: which way does each move?
  2. One additive in the collection tubes contains potassium and also binds calcium.
  3. Check each option against both results, the high one and the low one.

Review Blood collection tubes and order of draw

Problem 2 of 3

A panel drawn from the arm with a 5% dextrose infusion shows glucose 1,480 mg/dL, with sodium, potassium and creatinine all lower than this morning's values. The nurse asks you to run it again to be sure. What do you do?

A second run measures the same mixture of blood and dextrose fluid, so the results agree and both describe the tube.

Repeated the test on a line-contaminated specimen

A repeat measures the same diluted or spiked specimen again, so the second result agrees with the first and looks confirmed. Two matching results cannot show whether infusion fluid is in the tube. A new specimen from an acceptable site, or a line draw that clears the dead space, removes the contaminant.

The very high glucose with everything else diluted is the pattern of dextrose infusion in the tube. Only a specimen collected away from the infusion can show the patient's values.

Released numbers can be acted on whatever the comment says. A glucose from infusion fluid should not reach the chart as the patient's.

Hint
  1. Think about what is inside the tube, and whether a second run changes it.
  2. Dextrose infusion fluid is very high in glucose and holds no creatinine.

Review Specimen integrity and interference

Problem 3 of 3

A lavender EDTA tube for a complete blood count (CBC) arrives with a small clot visible when you tilt it. What do you do?

The cells and platelets caught in the clot are gone from the liquid that is left. Taking the clot out does not bring them back, and the counts would read low.

The counts would still be falsely low, and a clot can also block the analyzer's probe. A comment does not correct the numbers.

A clot in an anticoagulated tube has used up cells and platelets, so the specimen is unsuitable for a count. A new, well-mixed specimen is needed.

The analyzer may not flag a low count caused by a clot. The visible clot already shows the specimen is unsuitable.

Review Ordering, identification, and acceptability

Use it

  • A basic metabolic panel for a man in the medical intensive care unit (MRN 0038412) arrives at 06:10.
  • The unit nurse collected it at 05:40 and recorded no draw site.
  • His orders include 0.9% sodium chloride running continuously through a line in his left forearm.
  • Yesterday's panel was collected by venipuncture of the right arm.
  • Hemolysis, lipemia and icterus indices are within limits.
  • Both control levels on the chemistry analyzer were acceptable this morning.
TestResultPreviousReference intervalFlag
Sodium143 mmol/L139 mmol/LSep 22, 05:30135–145 mmol/L
Potassium3.2 mmol/L4.2 mmol/LSep 22, 05:303.5–5.1 mmol/LLow
Chloride115 mmol/L102 mmol/LSep 22, 05:3098–107 mmol/LHigh
Urea nitrogen (BUN)14 mg/dL18 mg/dLSep 22, 05:308–24 mg/dL
Creatinine0.83 mg/dL1.10 mg/dLSep 22, 05:300.74–1.35 mg/dL

Specimen: H 5, L 10, I 1. Collected 05:40 by the unit nurse; draw site not recorded.

Decision 1 of 3

Which explanation fits these results best?

EDTA carryover raises potassium. Here potassium fell, so EDTA does not fit.

Potassium, urea nitrogen and creatinine each fell by about a quarter as chloride rose. A change in the patient seldom moves unrelated analytes by the same fraction, and an even fall like this one is dilution.

Chloride rose toward saline's 154 mmol/L, sodium barely moved, and everything saline lacks fell by about a quarter. The line runs in the left arm, and no draw site was recorded.

Hemolysis raises potassium, and the hemolysis index is within limits. Neither fits.

Review Specimen integrity and interference

Decision 2 of 3

What do you do with the panel?

Sodium is diluted and spiked by the same saline. It lands inside the interval by chance and describes the mixture of blood and saline in the tube.

A repeat measures the same mixture of blood and saline and will agree with the first run. Agreement would not make it the patient's result.

Repeated the test on a line-contaminated specimen

A repeat measures the same diluted or spiked specimen again, so the second result agrees with the first and looks confirmed. Two matching results cannot show whether infusion fluid is in the tube. A new specimen from an acceptable site, or a line draw that clears the dead space, removes the contaminant.

The potassium and chloride would reach the chart as his values and could be treated. A comment does not stop that.

Every result in the tube carries the saline, so none of it is released. A new specimen from the arm without the infusion gives his real values.

Review Specimen integrity and interference

Decision 3 of 3

The redraw arrives labeled with his name and room number only. What do you do?

A name is one identifier, and a room number describes a place. The label does not tie the tube to one patient.

Matched a result to a patient by name alone

A name is one identifier, and two patients can share it. Giving or filing a result after matching the name alone can place one patient's value in another's record or give a caregiver the wrong result. Two person-specific identifiers, such as name with date of birth or medical record number, link a specimen to one patient.

The label needs two person-specific identifiers, such as name with date of birth or medical record number. Without them the tube cannot be accepted, however urgent the panel.

Confirming by phone afterward cannot show which patient's blood is in the tube. Identity is settled at the label, before testing.

Review Ordering, identification, and acceptability

The clue that settles this case is the shape of the change:

  • Chloride went up.
  • Sodium hardly moved.
  • Potassium, urea nitrogen and creatinine all went down by about 25%.

Only a fluid rich in sodium chloride and empty of everything else does that, and one was running in his arm. The contamination is in the tube, so a repeat would reproduce the same values. His real values come from a properly labeled redraw from the other arm.

Keep