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Can You Run an FBC on a Citrate Tube?

Normally, no — the FBC belongs in EDTA. The exception is a patient whose platelets clump in EDTA. When a citrate count is used, which numbers need the 1.1 correction, which do not, how fast it must be run, and how to report it.

Also called FBC in citrate tube, Citrate platelet count, Sodium citrate CBC, 1.1 correction

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The short answer

A full blood count is designed and verified for EDTA blood; the light-blue-top sodium citrate tube is for clotting tests. A routine FBC run on a citrate tube by mistake reads about 10% low and should be recollected into EDTA. A citrate count is used on purpose for one reason — platelets that clump in EDTA — and then mainly for the platelet count.

Read next Tube colours and the wrong-tube problemEDTA-dependent platelet clumping

Where the 1.1 factor comes from

Nine parts blood, one part citrate

A coagulation tube holds liquid 3.2% sodium citrate, one part to nine parts blood. Blood makes up nine-tenths of the mixture, so counts read about 10% low; multiplying by 10 ÷ 9, rounded to 1.1, corrects them.

Only when filled to the line

An underfilled tube holds proportionally more citrate, so the factor no longer fits. Reject underfilled tubes rather than correcting them.

Not the black-top ESR tube

ESR tubes hold four parts blood to one part citrate — a 20% dilution — and are not used for blood counts.

ESR methods and tubes

Your laboratory’s own factor

A CAP Today Q&A (December 2023) notes that the correction should be at least 10% and that each laboratory should validate its factor for the tube it uses and its usual delay before testing.

Which results to correct

Common practice is to report the platelet count — often with the white-cell count and MPV — from the citrate tube, and every other result from an EDTA tube taken at the same visit.

Based on CAP Today Q&A guidance (November 2014); follow your laboratory’s validated procedure.
ResultFrom a citrate tube
Platelet countMultiply by 1.1
White-cell count and absolute differential countsMultiply by 1.1
RBC, haemoglobin and haematocritMultiply by 1.1 if reported from citrate; usually taken from the EDTA tube instead
MCV, RDW and MPVNot corrected — they describe cell size, not concentration. Citrate and EDTA MPV values are not interchangeable
MCH, MCHC and differential percentagesNot corrected — they are ratios, so the dilution cancels out

Run it quickly

  • Citrate platelet counts fall with time. The CAP Today Q&A advises testing as soon as possible, ideally within one to three hours of collection.
  • Keep the sample at room temperature; do not refrigerate a sample taken for a platelet count.
  • Mix gently by inversion just before it goes on the analyser.
  • Take the citrate and EDTA tubes at the same venepuncture, citrate first in the order of draw, so the two can be compared.
  • White-cell morphology on a film made from citrate blood deteriorates; review films from the EDTA tube.

Read next Sample timing and storage

When citrate does not fix it

Citrate does not stop clumping in every patient. In a 2023 study of 151 patients with EDTA-dependent pseudothrombocytopenia in the Journal of Ayub Medical College, Pakistan, citrate resolved the clumping in only 47 (31%), and the authors did not support it as a routine alternative.

  • Vortex the EDTA tube for one to two minutes and rerun — CAP Today suggests this before recollecting.
  • Keep a fresh sample at 37 °C and run it straight away.
  • Take a capillary sample and run it at once, in pre-diluted mode if your analyser has one.
  • Estimate the count from a well-made film and report it as an estimate.
  • Use another anticoagulant only if your laboratory has validated it.

Read next Capillary and pre-diluted counts

What the report should say

  • Name the tube and the correction — for example, “platelet count from sodium citrate sample, corrected for dilution”.
  • Record what the film showed in the EDTA sample, such as “platelet clumps seen”.
  • In dengue follow-up, use the same method at every visit and say so, so the trend is not an artefact of changing tubes.
  • Flag the patient’s record so the next FBC request includes a citrate tube or a film review.
  • If your laboratory reports citrate counts routinely, include them in the analyser’s verification.

Read next Method verificationThe FBC in dengue monitoring

Related guides

Coagulation Analysers: PT/INR, APTT and Choosing a CoagulometerClotting tests run on different machines, from different tubes, under stricter sample rules than the FBC. What each test measures and which kind of analyser fits a clinic, an anticoagulation service or a hospital lab.Platelet Clumping: How to Catch a Falsely Low Platelet CountSome people’s platelets clump in the EDTA tube but not in their bodies, and the analyser reports a low count that is not real. How to recognise it, confirm it and report it so nobody acts on the wrong number.Blood Tube Colours and the Order of Draw, ExplainedWhich tube each test needs, the order to fill them in, and why the wrong cap colour — an FBC in a citrate tube, say — gives a wrong result rather than no result.

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Frequently asked questions

Can you do an FBC in a citrate tube?
Not as a routine — the FBC is verified for EDTA blood, and a citrate sample reads about 10% low. Laboratories use a citrate tube on purpose when a patient’s platelets clump in EDTA, then correct the platelet count by 1.1 and take the other results from EDTA.
Why is a citrate platelet count multiplied by 1.1?
The tube holds one part liquid citrate to nine parts blood, so the blood is diluted by a tenth. Multiplying by 10 ÷ 9, about 1.1, corrects for that, provided the tube was filled to the line.
How long is a citrate platelet count reliable?
Citrate platelet counts fall over time, so run the sample as soon as possible — ideally within one to three hours of collection — and keep it at room temperature.
Do you correct the MCV from a citrate tube?
No. MCV, RDW and MPV describe cell size, not concentration, and MCH and MCHC are ratios, so none is multiplied by 1.1.

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