Laboratories order vacuum blood collection tubes by cap colour, and one mixed-up pallet can spoil a month of results. Here is what each colour actually contains, the order of draw, and what to verify before you import them by the container.
Every cap colour is a different additive and a different test. A working map of SST, heparin, EDTA, citrate and ESR tubes - and the import checks that matter.
A vacuum blood collection tube is pre-filled with a measured vacuum and, usually, an additive. The cap colour is a worldwide convention for which additive - and therefore which tests - the tube serves.
| Cap colour | Additive | Common name | Used for |
|---|---|---|---|
| Gold / tiger-top | Clot activator + separator gel | Serum separator tube (SST) | Routine chemistry, serology, hormones |
| Plain red | Clot activator (no gel) | Plain serum tube | Serology, blood bank where gel interferes |
| Green | Sodium or lithium heparin | Heparin tube | Plasma chemistry, blood gases, trace elements (sodium heparin) |
| Lavender / purple | K2 or K3 EDTA | EDTA tube | CBC and haematology |
| Light blue | 3.2% sodium citrate (9:1) | Citrate tube | Coagulation - PT, APTT, INR |
| Black | 3.8% sodium citrate (4:1) | ESR tube | Erythrocyte sedimentation rate (Westergren) |
| Grey | Sodium fluoride + oxalate | Glucose tube | Glucose and lactate |
Two frequent mix-ups: the ESR tube is black-capped with a 4:1 citrate ratio - not the light-blue coagulation tube - and sodium heparin (not lithium) is required for trace-element work, because lithium heparin contaminates lithium assays and some metals panels.
Blood cultures first, then citrate (light blue), serum tubes (red/gold), heparin (green), EDTA (lavender), and fluoride (grey) last. The order prevents additive carry-over between tubes - labs audit suppliers whose tubes force deviations, so print it on your cartons.
Tubes travel best by sea in temperature-stable containers; air freight pressure cycles can disturb weak vacuums. Specify ISTA-tested export cartons and ask your supplier for transit validation data.
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A gold or tiger-top tube containing a clot activator and an inert separator gel. After centrifugation the gel forms a barrier between serum and cells, keeping the sample stable for chemistry, serology and hormone testing.
Green-capped sodium heparin tubes give plasma for chemistry, blood gases and especially trace-element testing, where lithium heparin would contaminate lithium and some metal assays.
Black. ESR (Westergren) tubes contain 3.8% sodium citrate at a 4:1 blood-to-additive ratio - different from the light-blue 9:1 citrate tube used for coagulation tests.
Blood cultures, then light blue (citrate), red/gold (serum), green (heparin), lavender (EDTA) and grey (fluoride) last, to prevent additive carry-over between tubes.
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