Also called: EDTA tube, Purple top tube, FBC sample collection

EDTA Blood Samples: Handling That Protects Every FBC Result

Most wrong FBC results start before the sample reaches the analyser. A step-by-step guide to the tube, the draw, the mix and the wait.

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Last updated: 8 October 2026

Why sample handling decides the result

Studies of laboratory errors consistently find that most of them arise in the pre-analytical phase — patient identification, collection, mixing, transport and storage — rather than inside the analyser. A well-maintained FBC machine will still report a wrong platelet count from a clotted tube. For a clinic, getting these steps right is the cheapest quality improvement there is.

The EDTA tube

K2 or K3 EDTA

FBC samples go into a lavender or purple-top tube containing dipotassium (K2) or tripotassium (K3) EDTA, which binds calcium so the blood does not clot. The International Council for Standardization in Haematology (ICSH) recommends K2 EDTA for blood counting; use the tube your analyser maker and lab have validated, and stick to it.

Fill to the line

The amount of EDTA is set for the tube’s stated volume. An under-filled tube leaves excess EDTA, which can shrink red cells, falsely lower the haematocrit and MCV, and disturb platelet counts. Let vacuum tubes fill until they stop drawing on their own.

Order of draw

When several tubes come from one venepuncture, follow your lab’s order of draw (commonly blood culture, citrate, serum, heparin, EDTA, then fluoride). EDTA carried over into a serum tube can falsely raise potassium and lower calcium in chemistry results.

Collection and mixing

Confirm the patient’s identity with two identifiers and label the tube beside the patient, never afterwards.
Keep tourniquet time short and avoid repeated probing, which can activate platelets and start clotting.
Use a suitable needle size; forcing blood through a narrow needle or syringe can haemolyse it.
Invert the tube gently straight after collection — typically 8 to 10 times, or as the tube maker specifies. Never shake it.
If transferring from a syringe, let the tube’s vacuum draw the blood in rather than pushing the plunger, and do it without delay.
Mix again by gentle inversion just before the sample is run, unless the analyser mixes tubes itself.

Common pre-analytical errors and what they do

Problem	Effect on the FBC	What to do
Clots or micro-clots	Falsely low platelets and sometimes white cells; clots can block the counting aperture	Inspect the tube before running; recollect if clotted
Platelet clumping	Falsely low platelet count; large clumps may be counted as white cells	Check flags and a blood film; recollect (see below)
Under-filled tube	Excess EDTA shrinks red cells and distorts HCT and MCV	Recollect to the fill line
Haemolysis	Red cells destroyed in the tube: low RBC and HCT, while haemoglobin is largely unchanged, so MCH and MCHC rise	Recollect with better technique
Cold agglutinins	Red cells clump at room temperature: low RBC, high MCV and very high MCHC	Warm the sample to 37 °C as your procedure states and rerun
Lipaemia or very high white-cell count	Turbidity can falsely raise haemoglobin, MCH and MCHC	Follow your analyser’s and lab’s correction procedure, or refer
Sample left too long	Red cells swell, raising MCV and HCT; the white-cell differential deteriorates	Run within the time the analyser maker and your lab validate
Drawn above a drip	Dilution lowers every count	Draw from the other arm; recollect

Platelet clumping and EDTA-dependent pseudothrombocytopenia

In a small proportion of people, antibodies make platelets clump in the EDTA tube even though their platelets behave normally in the body. The analyser then reports a falsely low platelet count — sometimes alarmingly low — often with a platelet-clump flag. In a dengue clinic, where platelet counts drive decisions, this matters. A blood film showing clumps confirms it; the lab may recollect in a sodium citrate tube and correct for the dilution. Note it on the patient’s record so the next sample is handled the same way.

Children and capillary samples

Microcollection tubes

Small EDTA microcollection tubes take a few hundred microlitres from a finger or heel prick. Fill to the marked range and mix while collecting, because capillary blood clots quickly.

Pre-diluted mode

Many clinic analysers can run a measured drop of capillary blood that is diluted before analysis. Use the maker’s pipette and diluent exactly, and make sure the mode is covered by your QC routine.

Technique

Wipe away the first drop and avoid squeezing hard — tissue fluid dilutes the sample and can activate platelets. Capillary values can differ slightly from venous ones, so keep to one sample type when following a patient.

Storage, transport and timing

Run FBC samples as soon as practical, and always within the time the analyser maker and your lab have validated.
Keep tubes at the temperature your procedure specifies; never leave them in sunlight or a hot car on the way to an outside lab.
Never freeze whole blood for an FBC — freezing destroys the cells.
Record the collection time on the request so a delayed result can be interpreted.
When samples are sent out, agree transport time and temperature with the receiving lab.

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Frequently asked questions

Why does an FBC go in a purple-top tube?

Lavender or purple is the usual cap colour for EDTA tubes, so staff can pick the right tube for an FBC at a glance. Check the label too, because cap colours can vary between tube makers.

How many times should an EDTA tube be inverted?

Typically 8 to 10 gentle inversions straight after collection, but follow the tube maker’s instructions. Shaking can damage cells and cause haemolysis.

Can a clotted sample still be run for an FBC?

No. Clots trap platelets and white cells, so the counts come out falsely low, and clots can block the analyser. Recollect the sample.

What does a very high MCHC usually point to?

Often a sample problem rather than a patient problem — cold agglutinins, haemolysis and lipaemia are common causes. Check the sample and follow your lab’s procedure before the result is reported.

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