UK Blood Tubes: Order of Draw, Additives and Local Colour Codes

Blood tubes are one of those placement topics that can look simple until you are standing beside a patient with several requests and a tray of different bottles.

The important lesson is not to memorise one universal UK colour sequence. Blood tube cap colours and even the precise order used can vary between laboratories, manufacturers and NHS organisations. Your local pathology guidance is therefore the authority for the tubes used in your clinical area.

What you do need to understand is why different tubes contain different additives, why the order of draw matters and how to check the correct specimen before collecting blood.

Why does the order of draw matter?

When several specimens are collected during one venepuncture, small amounts of an additive from one tube can potentially contaminate the next tube. This is called additive carryover.

That matters because additives deliberately alter the blood specimen. EDTA, citrate and heparin, for example, are used for different analytical purposes. Contamination can therefore produce misleading laboratory results.

Order-of-draw guidance is designed to reduce this risk.

The first rule: learn the additive, not only the colour

Students naturally remember tubes by colour, but colour alone is not sufficiently reliable for clinical practice.

Different UK laboratories use different collection systems, so when you move between placements, do not assume that the cap colour you learnt elsewhere means exactly the same thing. Read the tube label and follow the local laboratory specimen guide.

A useful clinical order to understand

Although local systems vary, a commonly encountered sequence is:

1. Blood cultures, when required
2. Citrate/coagulation specimen
3. Serum or serum-separator specimen
4. Heparin specimen
5. EDTA specimen
6. Fluoride/oxalate specimen

This is why Bleepbook is deliberately not presenting one colour mnemonic as “the UK order”. Check the order used by your local laboratory every time you work in a new organisation.

Blood cultures: aerobic and anaerobic bottles

Blood cultures are used to investigate suspected bloodstream infection. Where cultures are required alongside routine blood specimens, they are generally collected first in the order of draw.

Adult blood-culture collection commonly uses a paired set of bottles:

• Aerobic blood-culture bottle — supports recovery of aerobic and facultative organisms according to the culture system used.
• Anaerobic blood-culture bottle — supports recovery of anaerobic and facultative organisms according to the culture system used.

The volume of blood cultured is one of the most important factors affecting diagnostic yield. NHS England's blood-culture pathway recommends two sets for adults with suspected sepsis, comprising two aerobic and two anaerobic bottles, with 8–10 mL per bottle, while also emphasising that the manufacturer's optimum volume for the local system should be followed.

Which bottle is filled first?

The sequence depends on the collection method and local system. With a winged or butterfly collection set, NHS guidance commonly directs the aerobic bottle first because air within the tubing would otherwise enter the anaerobic bottle. Some NHS guidance for syringe collection specifies the anaerobic bottle first. Follow the manufacturer's instructions and your organisation's microbiology procedure rather than applying one bottle order to every collection method.

Blood-culture bottle cap colours are manufacturer and local-system dependent. Do not identify an aerobic or anaerobic bottle by colour alone; check the bottle label.

Collection quality matters

Blood-culture contamination can produce false-positive results and may contribute to unnecessary antimicrobial treatment and further investigation. Patient identification, appropriate skin antisepsis, disinfection of the bottle septum, correct blood volume and aseptic technique are therefore essential.

Blood cultures should ideally be obtained before antimicrobial therapy is started when this is clinically appropriate, but collection should not delay urgent antimicrobial treatment when treatment is indicated.

Existing vascular access devices should not be used routinely for blood cultures simply because they are convenient. When line-associated infection is suspected, paired line and peripheral cultures may be required according to local microbiology guidance.

Citrate: coagulation testing

Additive: sodium citrate.

Common uses: coagulation investigations such as PT, INR and APTT, depending on the laboratory request.

Citrate binds calcium reversibly, preventing coagulation within the specimen. The blood-to-anticoagulant ratio is important, which is why the tube needs the correct fill volume.

Clinical reasoning: an underfilled citrate tube changes the blood-to-citrate ratio and may make the sample unsuitable for accurate coagulation testing.

If a butterfly collection set is being used, follow local instructions about whether the tubing needs to be primed or a discard tube is required before the citrate specimen.

Serum and serum-separator tubes

Serum specimens allow the blood to clot before the serum is separated for analysis.

Depending on the collection system, you may encounter a plain serum tube or a serum-separator tube containing a clot activator and separator gel.

Common uses can include: biochemistry, immunology, serology, hormones and therapeutic drug monitoring, but the correct specimen depends on the individual test and laboratory.

Do not use the rule “gold does everything”. Some investigations require a different specimen, and the local pathology test directory should be checked when you are unsure.

Heparin tubes

Additive: commonly lithium heparin or sodium heparin, depending on the tube and investigation.

Common use: plasma biochemistry.

Heparin inhibits coagulation, allowing plasma rather than serum to be analysed.

Do not assume that every investigation associated with cardiac care automatically belongs in a particular coloured tube. For tests such as troponin, use the specimen specified by your laboratory.

EDTA tubes

Additive: EDTA (ethylenediaminetetraacetic acid).

Common uses: haematology investigations such as full blood count (FBC), platelets, reticulocytes and HbA1c, depending on local laboratory requirements.

EDTA binds calcium and prevents clotting, helping preserve cellular components for haematological analysis.

Why order matters: EDTA carryover can interfere with other specimens. This is one reason EDTA is normally collected after serum/heparin specimens in order-of-draw sequences.

What about group and screen or crossmatch?

Transfusion specimens deserve particular attention. Laboratories may use a dedicated EDTA tube, often with a different cap colour from the routine haematology EDTA tube. Patient identification, specimen labelling and transfusion sampling procedures must follow the organisation's transfusion policy exactly.

Do not assume that a routine EDTA tube is interchangeable with the tube specified by the transfusion laboratory.

Fluoride/oxalate tubes

Additives: commonly sodium fluoride with an anticoagulant such as potassium oxalate, depending on the collection system.

Common uses: glucose and, in some laboratories, lactate or alcohol testing.

Fluoride inhibits glycolysis, helping preserve glucose in the specimen. These tubes are generally positioned late in the order of draw.

What about trace-element and specialist tubes?

Specialist investigations may require trace-element tubes, acid-citrate-dextrose tubes or other collection containers. Their additives, materials and position in the order of draw depend on the laboratory system and test.

If you have not seen a tube before, stop and check the pathology specimen requirements rather than choosing by colour.

Five things to check before collecting a specimen

1. Correct patient: identify the patient using your organisation's approved identifiers.
2. Correct investigation: confirm which blood tests have actually been requested.
3. Correct specimen: check the required tube/additive and volume.
4. Correct order: follow your local laboratory's order of draw.
5. Correct labelling and handling: label, mix, store and transport the specimen according to local policy.

Mixing tubes: invert, do not shake

Tubes containing additives need appropriate mixing after collection. They should be gently inverted according to the manufacturer's and local laboratory instructions rather than vigorously shaken.

The required number of inversions can vary by tube and manufacturer, so check the collection system used in your clinical area.

Common reasons specimens are rejected

Knowing the tube colours is only one part of obtaining a usable blood specimen. Samples may be rejected or compromised because of:

• incorrect or incomplete patient identification;
• wrong specimen container;
• insufficient volume, particularly for ratio-dependent specimens such as citrate;
• clotted anticoagulated specimens;
• haemolysis;
• contamination;
• incorrect labelling;
• inappropriate storage or delayed transport.

A placement example

Your patient has requests for coagulation studies, renal function and a full blood count. On your previous placement you used a light-blue citrate tube, gold serum tube and purple EDTA tube.

You arrive on a new placement and the tube colours look different.

The safe response is not to recreate the colour sequence from memory. Check the local pathology guide or approved collection chart, identify each tube by its label and additive, confirm the required order and ask an appropriately trained member of staff if you remain unsure.

Quick learning summary

Blood cultures: generally collected first when required; adult sets commonly contain aerobic and anaerobic bottles. Blood volume is critical, and bottle order depends on the collection method and local system.

Citrate: coagulation testing; correct fill is particularly important.

Serum: blood clots before serum is separated; used for many chemistry, immunology and serology investigations.

Heparin: anticoagulated plasma; commonly used for biochemistry.

EDTA: commonly used for haematology; collect in the locally specified position to avoid additive contamination.

Fluoride: inhibits glycolysis and is commonly used for glucose specimens.

Most important: colours and local sequences vary. Read the tube or bottle label and follow your own laboratory's current guidance.

References and further reading

World Health Organization (2010) WHO guidelines on drawing blood: best practices in phlebotomy. Geneva: World Health Organization. Available at: WHO guidelines on drawing blood 

NHS England and NHS Improvement (2022) Improving the blood culture pathway. Available at: NHS England blood culture pathway 

Royal United Hospitals Bath NHS Foundation Trust (n.d.) Blood cultures. Available at: RUH Blood Cultures 

NHS Greater Glasgow and Clyde (n.d.) Blood cultures. Right Decisions. Available at: NHSGGC blood culture guidance 

Royal United Hospitals Bath NHS Foundation Trust (n.d.) Phlebotomy guide: order of draw. Available at: RUH Pathology 

NHS Lothian (n.d.) Order of draw. Right Decisions. Available at: NHS Lothian order of draw