Venepuncture and blood cultures
Clean, let it dry, cultures first, then tubes in the right order. Label at the bedside. Two sets from two sites before antibiotics.
A 70-year-old woman has a temperature of 39.1 °C and rigors. The nurse asks for blood cultures before the first dose of antibiotics. Her only good vein is in the left antecubital fossa, where she has a cannula with fluids running.
Where do you take the cultures from, how do you avoid contamination, and in what order do the tubes go?
- Cultures before antibioticsBut never delay antibiotics in sepsis for more than a short wait.
- Fresh siteNot from an existing cannula. Not from an arm with a drip running.
- Clean the skin2% chlorhexidine in 70% alcohol for 30 seconds, let it dry 30 seconds, don't touch again.
- Clean the bottle topsAlcohol wipe, let dry.
- Fill the right volume8–10 mL per bottle in adults. Aerobic first if using a butterfly.
- Order of drawCultures, citrate (blue), plain or gel (red or gold), heparin (green), EDTA (purple), fluoride (grey).
- Label at the bedsideName, time, site, any antibiotics given.
- Arterial puncture
- Pain shooting down the arm (nerve)
- Fainting
- Sepsis: start antibiotics within 1 hour
- Contaminated cultures (skin organisms)
- Sampling above a drip (diluted results)
- Haemolysed potassium
- Wrong patient labelling
- Underfilled bottles
| Volume per bottle (adult) | 8–10 mL |
| Sets | 2 sets from separate sites for suspected bacteraemia or endocarditis |
| Endocarditis | 3 sets from separate sites, spaced out, before antibiotics if stable |
| Skin prep | Chlorhexidine 2% in alcohol 70%, 30 seconds plus drying |
| Tourniquet | Under 1 minute |
| Needle | 21G (green) routine |
Check every dose against a current drug reference and your hospital protocol.
On this page
Choosing the site
First choice, on the arm without a drip
When the antecubital veins are poor
Above a running drip, fistula arm, lymphoedema or node-cleared side, through an existing cannula (except at insertion), infected skin
The brachial artery and median nerve lie deep and medial in the antecubital fossa, under the bicipital aponeurosis. Prefer lateral veins and stay superficial.
Technique
- Identify the patientName and number against the request.
- Tourniquet7–10 cm above the site, less than a minute.
- CleanChlorhexidine in alcohol, 30 seconds, dry.
- InsertBevel up, 15–30°, skin stretched below.
- FillTubes in the correct order; invert each gently.
- FinishRelease tourniquet before removing the needle, press with gauze, sharps straight in the bin, label at the bedside.
Blood cultures without contamination
- Hand hygiene, clean trayNon-touch technique.
- Bottle topsRemove caps, clean with alcohol, let dry.
- SkinChlorhexidine 2% in alcohol, 30 seconds, dry. Do not re-palpate.
- Sample8–10 mL into each bottle. Butterfly: aerobic first (air in the tubing). Needle and syringe: anaerobic first.
- Then other tubesCultures always come first.
- DocumentTime, site, temperature, antibiotics already given.
Two sets from separate sites help separate true bacteraemia from contamination: a skin organism (coagulase-negative staphylococcus) in only one set is usually a contaminant.
Order of draw
| Order | Tube | Common tests |
|---|---|---|
| 1 | Blood culture bottles | Cultures |
| 2 | Blue (citrate) | Clotting (fill to the line) |
| 3 | Red or gold (plain or gel) | U&E, LFT, CRP, lipids |
| 4 | Green (heparin) | Some biochemistry |
| 5 | Purple (EDTA) | FBC, HbA1c, group and save (check local) |
| 6 | Grey (fluoride) | Glucose, lactate |
EDTA carried into a biochemistry tube binds calcium and gives a falsely high potassium and low calcium.
Avoiding common errors
- Haemolysis: small needles, vigorous shaking, long tourniquet time, delayed processing. Gives falsely high potassium.
- Drip arm: dilutes results (low Hb, odd electrolytes, high glucose if on dextrose).
- Labelling: always at the bedside, never pre-labelled.
- Group and save: usually needs handwritten labelling and a second sample per local rules.
Common mistakes
- Taking cultures from an existing cannula.
- Re-touching the skin after cleaning.
- Underfilling culture bottles.
- Delaying antibiotics in septic shock for cultures.
- Pre-labelling tubes.
- Sampling above a running drip.
Frequently asked questions
What is the order of draw for blood tubes?
Blood cultures, then citrate (blue), plain or gel (red or gold), heparin (green), EDTA (purple) and fluoride (grey).
How much blood goes in each culture bottle?
8–10 mL per bottle in adults.
Why take two sets of blood cultures?
To separate true infection from skin contamination, and to increase the chance of finding the organism.
What causes a falsely high potassium?
Haemolysis, a long tourniquet time, delay in processing or EDTA contamination.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. Which is filled first when taking cultures and other bloods?
Answer: Culture bottles. Cultures first to reduce contamination.
- Fluoride tube: Last.
- Heparin tube: Middle.
- Citrate tube: After cultures.
- EDTA tube: Last-but-one.
2. A potassium of 7.5 with low calcium in a well patient. What sampling error is likely?
Answer: EDTA contamination. EDTA chelates calcium and contains potassium.
- Fluoride contamination: No.
- Delay in analysis: Possible high K.
- Haemolysis only: Causes high K but not low calcium.
- Wrong patient: Possible but EDTA fits.
3. How much blood goes in each adult blood culture bottle?
Answer: 8–10 mL. Proper filling increases detection of bacteraemia.
- 3 mL: Too little.
- 1 mL: Too little.
- Any amount: Volume matters.
- 20 mL: Too much.
4. After a supracondylar fracture of the humerus, a child cannot flex the index finger when making a fist (pointing index, 'hand of benediction') and the thenar muscles are weak. Which nerve is injured?
Answer: Median nerve at the elbow. A median nerve injury at the elbow paralyses the forearm flexors it supplies (FDS, lateral FDP, FPL) and thenar muscles. On making a fist, the index (and middle) finger stays straight. Injury at the wrist (carpal tunnel) spares forearm flexors, causing only thenar weakness and sensory loss. Supracondylar fractures also risk the brachial artery (Volkmann's contracture).
- Ulnar nerve at the elbow: Clawing of ring and little fingers.
- Radial nerve in the spiral groove: Wrist drop.
- Median nerve at the wrist only: Spares the long finger flexors.
- Musculocutaneous nerve: Weak elbow flexion.
5. Which muscles of the hand are supplied by the recurrent (motor) branch of the median nerve?
Answer: APB, FPB and opponens. The median nerve supplies the 'LOAF' muscles: the Lateral two lumbricals (index and middle fingers), Opponens pollicis, Abductor pollicis brevis and Flexor pollicis brevis (superficial head). The thenar muscles are supplied by the recurrent branch, which curls around the distal edge of the flexor retinaculum and is vulnerable to superficial lacerations ('million-dollar nerve'), causing loss of opposition. All other intrinsic hand muscles—hypothenar muscles, interossei, medial two lumbricals, adductor pollicis, deep head of FPB and palmaris brevis—are supplied by the ulnar nerve.
- Adductor and all interossei: Ulnar nerve.
- Hypothenar muscles: Ulnar nerve.
- All four lumbricals: Only the lateral two are median.
- Palmaris brevis: Ulnar nerve.