The Correct Order of Draw in Blood Collection: Why It Matters
, by Andrew Odgers, 9 min reading time
Pre-analytical Quality
The Correct Order of Draw in Blood Collection: Why It Matters
The order in which blood collection tubes are filled during a single venepuncture is not arbitrary. Additive carryover from one tube to the next can interfere with laboratory results in ways that are clinically significant and difficult to detect after the fact. Understanding the correct order and the rationale behind it is a fundamental competency for anyone performing phlebotomy.
Why order of draw matters
The carryover problem explained
Additive contamination between tubes
Each tube contains a different additive, and as the needle passes between tubes a small amount of additive from the previous tube may be carried over into the next on the needle tip or in the residual blood in the needle hub. The standard order of draw is designed to sequence tubes so that any carryover from one tube to the next has the least possible effect on the results in the subsequent tube. Departing from this sequence can introduce additive contamination that invalidates specific tests, sometimes subtly enough that the error is not immediately apparent.
The coagulation tube is particularly vulnerable
The citrate coagulation tube is the most vulnerable to carryover contamination. EDTA from a purple haematology tube carried into the light blue citrate tube chelates additional calcium and produces falsely prolonged coagulation times. Tissue thromboplastin released from the venepuncture needle, which is present in the first few drops of blood, can activate coagulation pathways in the citrate tube if it is drawn first. For this reason, a discard tube or blood culture collection typically precedes the citrate tube.
The standard UK order of draw
Sequence, tube type, and rationale for each position
This sequence reflects current CLSI and UK phlebotomy guidelines. Always follow your laboratory's current SOP, which may specify minor variations.
- 1. Blood culture bottles (aerobic then anaerobic). Drawn first to minimise skin contamination from the puncture site. The first draw carries the highest risk of skin bacteria entering the sample; bottles filled first get the freshest blood with the lowest contamination risk.
- 2. Sodium citrate light blue (coagulation tube). Drawn before tubes containing anticoagulants that would contaminate the fixed citrate ratio. When only a coagulation tube is needed and no blood culture is drawn, a discard tube (plain red) should be drawn first to clear tissue thromboplastin from the needle before the citrate tube is filled.
- 3. Plain red or gold SST (serum tube). Clot activators and gel do not significantly affect subsequent tubes. The SST is positioned after citrate to avoid EDTA or heparin carryover into the serum sample.
- 4. Green lithium heparin (plasma tube). Heparin carryover into subsequent tubes is less problematic than EDTA carryover for most common assays. Positioned after serum tubes.
- 5. Purple EDTA (haematology tube). EDTA is the additive most likely to cause significant interference if carried over into other tubes. Positioned near the end of the sequence to protect preceding tubes.
- 6. Grey fluoride oxalate (glucose tube). Drawn last or near last. Fluoride carryover has minimal impact on most preceding tube results.
- Paediatric collections and single-tube draws. When only one or two tubes are being drawn, apply the same order principles. When drawing only a coagulation tube, always use a discard tube first if no prior tube has been drawn.
All tube types in stock
Blood collection tubes in every additive type for correct order of draw
Charles Medical supplies all tube types needed for the full order of draw sequence. Next-day UK delivery.
For tube colour identification to support order of draw practice, see Understanding Blood Tube Colours and What Each One Means.
Part of the hub
Back to the Blood Collection Tubes Knowledge Hub
This article is part of our complete blood collection tube knowledge base, covering tube types, colours, additives, order of draw, pre-analytical errors, disposal, and everything phlebotomists and laboratory staff need to know.
Keep reading
Related guides in this hub
Understanding Blood Tube Colours and What Each One Means provides the colour-to-additive reference for all tubes in the sequence. Common Mistakes in Blood Tube Usage covers order of draw errors alongside other pre-analytical errors. And Why the Right Tube Matters for Each Blood Test explains the downstream consequences of carryover errors.
Frequently asked
Order of draw questions answered
Why is the coagulation tube drawn second rather than last?
The coagulation tube is drawn early in the sequence to avoid EDTA carryover from the haematology tube, which would chelate additional calcium and falsely prolong coagulation times. It is drawn after blood cultures to ensure blood culture bottles get the freshest, lowest-contamination blood.
Do I need a discard tube before a coagulation tube?
When only a coagulation tube is being drawn with no prior blood culture, a discard plain red tube should be drawn first to clear tissue thromboplastin from the needle tip. Most laboratories specify whether a discard tube is required in their local phlebotomy SOP. If in doubt, use a discard tube.
Does order of draw matter for a single tube collection?
For a single tube, there is no sequence to manage. However, if that single tube is a coagulation tube and no blood culture or discard tube precedes it, the first blood into the needle may contain tissue thromboplastin that can activate coagulation pathways in the sample. Follow local SOP guidance on whether a discard tube is required for single coagulation draws.
What happens if I draw tubes in the wrong order?
Results may be affected by additive carryover in ways that are difficult to detect. The most common clinically significant error is EDTA carryover into a coagulation tube producing falsely prolonged PT and APTT. If you suspect an order of draw error, inform the laboratory. They may be able to identify likely interference or advise on whether recollection is needed.