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This page describes products and general use; it is not medical advice.
The additive in a light blue top (blue top) blood collection tube is buffered sodium citrate, an anticoagulant that works at a fixed ratio of one part additive to nine parts blood. This page explains what that additive is, why the fill line matters, what the manufacturer specification looks like for one common tube, and how to avoid the handling mistakes that change the ratio. It describes the product only and does not interpret any coagulation test result.
Who uses it: phlebotomists and clinic or laboratory staff who collect and handle coagulation specimens, buyers who stock collection supplies, and students learning tube types.
What the blue top tube additive is
The additive is sodium citrate, a liquid anticoagulant placed in the tube before it is shipped. The closure colour is the visual cue: the tube listed in the BD Vacutainer citrate range (catalogue 363083) has a light blue BD Hemogard closure and contains buffered sodium citrate at 3.2% (0.109 M). Searches for “light blue top tube additive” and “blue top tube additive” describe the same product class: a light blue closure means a citrate tube.
The ADLM (Academy of Diagnostics & Laboratory Medicine) describes these as “blue top 3.2% sodium citrate tubes” that “must have anticoagulant in ratio of 9 parts of blood to 1 part anticoagulant (9:1 ratio)”. The citrate is therefore a defined proportion of the tube contents, not a trace additive.
How the 9:1 ratio and the fill line work
A 9:1 ratio means nine parts blood to one part citrate, so citrate makes up one part in ten of the finished mixture. The tube is built so that a correct draw reaches that proportion. This is why the tube must be filled to its fill indication rather than “close enough”.
- Minimum fill. ADLM states that sodium citrate tubes “require at least 90% fill”.
- Underfilled tube. Too little blood means too much citrate relative to blood. ADLM describes the result as “a dilutional effect, excess calcium binding and falsely prolonged clotting times”.
- Overfilled tube. ADLM notes that overfilling “theoretically risks clotting, as the ratio of additive anticoagulant to whole blood may be insufficient”.
- No topping up. ADLM states that “2 partially filled blue tops cannot be combined to make 1 full tube”.
ADLM points to the CLSI guideline H21-A5 (2008) for collection and handling of coagulation specimens.
Types and variants
| Variant | Typical use | Key difference | Source |
|---|---|---|---|
| Light blue closure, buffered sodium citrate 3.2% (0.109 M) | Coagulation specimen collection (as described by ADLM) | Citrate at 3.2% with a 9:1 blood to additive ratio | BD product page 363083; ADLM |
Read the specification printed on the tube you actually stock; this table lists the variant documented in the sources below.
Sizes and draw volume
Sodium citrate tube sizes and volumes depend on the product. The BD product page for catalogue 363083 gives the following:
| Specification | BD Vacutainer citrate tube 363083 |
|---|---|
| Additive | Buffered sodium citrate, 0.109 M (3.2%) |
| Tube size | 13 x 75 mm |
| Draw volume | 2.7 mL |
| Closure | BD Hemogard, light blue |
| Material | Plastic |
| Packaging | 100 tubes per box, 1,000 per case |
For tube volume in practice, the draw volume above is the nominal figure and the 90% minimum from ADLM is the floor.
How to choose
Two criteria come from the cited guidance:
- Match the additive to the collection guidance your laboratory follows (for example CLSI H21-A5, as cited by ADLM), which for blue top tubes describes 3.2% sodium citrate.
- Handle the tube according to the manufacturer’s instructions for use that come with it.
Beyond these, the specification printed on the tube and your laboratory’s own collection protocol decide.
Safe use and handling basics
- Follow your laboratory’s collection protocol and the manufacturer’s instructions for use for the tube in hand.
- Treat the fill indication as a requirement of the product, not a suggestion.
- Mixing is covered by the manufacturer’s instructions for the tube.
Common mistakes
- Short fill. A tube filled below 90% falls outside the stated requirement and breaks the 9:1 ratio (ADLM).
- Combining two partial tubes. ADLM states that this cannot be done.
- Assuming overfill is harmless. Overfilling alters the ratio the other way (ADLM).
- Judging the tube by colour alone. Confirm concentration and size against the label and the manufacturer specification.
Frequently asked questions
What is the additive in a blue top tube?
Buffered sodium citrate, an anticoagulant. On the BD tube listed on this page it is 3.2% (0.109 M).
What is the light blue top tube additive?
The same thing: light blue and blue top refer to the citrate tube closure. Sodium citrate is the additive.
What ratio of blood to additive should a sodium citrate tube have?
Nine parts blood to one part anticoagulant (9:1), per ADLM.
What are common sodium citrate tube sizes and volumes?
One sourced example is BD catalogue 363083: 13 x 75 mm with a 2.7 mL draw volume. Other sizes vary by product, so read the specification on the tube.
How full must the tube be?
At least 90% full, according to ADLM, and partially filled tubes cannot be combined.
Related CASRAI pages
- Phlebotomy lab supply checklist
- Specimen labeling error prevention
- Specimen collection kit standardization
- Lab equipment and supplies procurement guide
- Biospecimen
Sources
- BD, Vacutainer citrate tubes, catalogue 363083, https://www.bd.com/en-us/products-and-solutions/products/product-print-page.363083, checked 2026-10-30.
- ADLM, “Pre-Analytic Variables in Coagulation Testing: Why do they matter” (2023), https://myadlm.org/science-and-research/scientific-shorts/2023/pre-analytic-variables-in-coagulation-testing-why-do-they-matter, checked 2026-10-30.
- CLSI, H21-A5 (2008), sample collection and handling guideline, as cited by ADLM (the guideline itself is not reproduced here).
