Module 2 of 2 · 75 minutes
Order of draw, sample quality and when it goes wrong
By the end of this module you will be able to
- Apply the order of draw and explain the carryover it prevents
- Identify the causes of haemolysis and prevent them
- Respond correctly to nerve pain, arterial puncture and haematoma
- Follow the sharps and needlestick injury procedure without delay
Work through it
1 interactive for this module, built on the WAJD Teach engine. Nothing moves until you ask it to, and every one has a written version if you would rather read it.
Amara Order of draw. Be honest with me, is this real or is it laboratory fussiness?
Nadia It is real and I can prove it in one example. The EDTA tube, the purple one for a full blood count, contains potassium as part of its additive. Draw it before the tube for urea and electrolytes and a trace carries over on the needle.
Amara And that shows up?
Nadia As a high potassium. Which is a number clinicians take extremely seriously, because a genuinely high potassium can stop a heart. So the patient may get treated, or at minimum gets recalled, re bled and frightened. All because two tubes went on the needle in the wrong sequence.
Amara Give me the order then.
Nadia Blood cultures first, because they must stay sterile. Then citrate, the coagulation tube. Then serum. Then heparin. Then EDTA. Then fluoride oxalate for glucose last.
Amara You said the coagulation tube has to be full.
Nadia Completely full, and this one catches people out. Citrate works on a fixed ratio of blood to additive. Underfill it and the ratio is wrong, so the clotting time is wrong, and somebody may be adjusting a warfarin dose on it. The laboratory rejects underfilled coagulation tubes and they are right to.
Amara Now tell me about haemolysis, because I hear that word constantly.
Nadia Red cells bursting and leaking their contents into the plasma. And the reason it matters is the same reason as before: potassium lives inside red cells. Burst them and you get a high potassium that belongs to your technique, not to the patient.
Amara Can I see it happening?
Nadia No. That is exactly what makes it dangerous. It is invisible to you and obvious to the laboratory. Which means the only defence is technique: do not use a needle finer than the job needs, do not yank a syringe plunger, let the vacuum tube fill at its own pace, and invert tubes gently rather than shaking them like a cocktail.
Amara Right. The patient suddenly says there is a sharp pain shooting down to their fingers.
Nadia Needle out. Immediately. Not adjust, not wait to see, not one more try. That description means nerve, and the difference between a temporary problem and a permanent one is often how fast the needle came out.
Amara And I do not try the other vein in that arm?
Nadia Not that arm. Document it, report it, tell somebody senior. I know there is a pull to finish the job. Resist it.
Amara What if the blood is bright red and coming in fast?
Nadia Think artery. Out, and firm pressure for a full five minutes, longer if they are on anticoagulants, and you stay with them until it has genuinely stopped. Then tell the clinical team. Arterial punctures can bleed deep into the arm afterwards, and that is compartment syndrome territory.
Amara Last one. I stick myself.
Nadia Encourage it to bleed, wash under running water, do not scrub it, cover it, report it now. Not at the end of your list. Now.
Amara Why so absolute about the timing?
Nadia Because post exposure prophylaxis for HIV works best measured in hours. A needlestick reported the next morning is a needlestick reported too late. And people delay for the worst possible reason, which is embarrassment. Nobody has ever been disciplined for reporting one promptly. People have been very seriously harmed by sitting on one.
The written material
Why the order of draw exists
Tubes contain different additives, and a small amount of additive is carried over on the needle from one tube to the next. The order exists so that any carryover is harmless rather than result changing.
The classic failure is drawing the EDTA tube, which contains potassium, before the tube used for urea and electrolytes. Potassium carries over, the potassium result comes back high, and a patient can be treated for a hyperkalaemia they never had. The order is not laboratory bureaucracy. It is the reason the number is real.
Haemolysis, the invisible error
Haemolysis is red cells rupturing and spilling their contents into the plasma. It is invisible to you at the bedside and obvious to the laboratory, and it matters most because potassium sits inside red cells. A haemolysed sample reports a high potassium that belongs to the tube, not the patient.
Nearly every cause is mechanical and preventable: a needle too fine for the draw, excessive suction from pulling a syringe plunger hard, shaking tubes instead of inverting them gently, drawing through a difficult line, or frothing at the needle hub.
- Invert tubes gently the stated number of times; never shake
- Avoid an unnecessarily fine needle for a routine draw
- Let vacuum tubes fill at their own rate rather than forcing flow
- Let alcohol skin prep dry fully before puncture
Complications and what you do about them
Sharp, shooting or electric pain travelling down the arm suggests nerve involvement. Remove the needle immediately. Do not reposition to see whether it settles, and do not reattempt on that arm. Document it and report it, because early recognition is what separates a transient injury from a lasting one.
Bright red blood that pulses or fills the tube unusually fast suggests arterial puncture. Remove the needle and apply firm pressure for at least five minutes, longer if the patient is anticoagulated, and do not leave them until bleeding has stopped. Tell the clinical team.
A haematoma forming under the skin during a draw means the needle is no longer wholly within the vein. Stop, remove, apply pressure. Bruising afterwards is common and usually harmless, but a swelling that appears during the draw is a reason to stop, not to persist.
Sharps and needlestick injury
The Health and Safety (Sharp Instruments in Healthcare) Regulations 2013 require safer sharps devices to be used where reasonably practicable, and prohibit recapping. Activate the safety mechanism before the needle leaves your hand and dispose at the point of use, into a sharps bin you have already positioned within reach.
If you sustain a needlestick injury, the sequence is fixed and the timing matters: encourage bleeding, wash under running water without scrubbing, cover, report immediately, and attend occupational health or accident and emergency the same day. Post exposure prophylaxis for HIV is most effective when started within hours, so a needlestick reported the following morning is a needlestick reported too late.
- Never recap a used needle
- Activate the safety mechanism immediately, at the bedside
- Position the sharps bin within arm's reach before you start
- Report the same day: prophylaxis is time critical
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