Guide to Collecting Blood
- Venipuncture, or phlebotomy, is the process of collecting blood for testing. This section outlines the process for taking blood, with pitfalls and clinical pearls.
- The following equipment is required for collecting blood:
Identifying a Vein
- Apply the tourniquet and feel for a straight, firm, round, elastic, non-pulsatile vein.
- Potential sites for venepuncture include the antecubital fossa, the cephalic or basilic veins in the forearm, or the small veins on the back of the hand.
Tips for Choosing a Vein
- Use the nondominant arm if possible
- Try to use the antecubital fossa where possible
- Absolutely do not use the same arm as an AV fistula
- Don’t use the same arm as a past lymph node dissection or mastectomy
- Don’t use a limb affected by stroke
- Don’t insert a needle through a burn, oedema, haematoma or infected tissue
- If testing coags, do not take from the same limb as a heparin infusion
- If an infusion is running, either stop the infusion prior to taking the sample or use another limb
Methods to Assist with Finding a Vein
- If you're having difficulty finding a vein, warm the area using blankets or a heat pack. Tap the vein repeatedly, which may make it more prominent. Ask the patient to pump their fist repeatedly, or hang the limb over the bed to increase the blood flow.
- Certain devices may also be used to assist with finding a vein. Ultrasound-guided blood collection can be performed if no veins are identifiable on inspection and palpation. Certain hospitals have vein viewers which use technologies such as infrared light to highlight veins.
Collecting the Sample
Order of DrawThe order of blood collection is important, as chemicals from one tube can cross-contaminate another tube and affect the results. The optimal order of collection is:
- Blood culture tubes
- Citrate tube - coags
- SST tube - chemistry, immunology, serology, blood bank
- Heparinised tube - lithium level, ammonia level
- EDTA - haematology, blood bank
- Acid-citrate-dextrose - HLA typing, paternity testing, DNA studies
- Glucose inhibitor tube
Likely Cause What to Do No flashback Needle is not in the vein Advance further, or pull back slightly then reinsert slightly more medially / laterally Flashback but no blood coming out Needle penetrated too far, or vein collapsed Pull back slightly, adjust the angle of the needle against the skin, pull back and try again Bright red blood pulsating out Needle is in an artery Remove the needle and apply pressure for at least three minutes Haematoma Bleed into the subcutaneous space Remove the needle and apply pressure to the area
- If blood leaks from the vein into surrounding tissue, this can result in red / purple discolouration, swelling and tenderness. If this occurs, then apply a pressure bandage to the site.
- Bruising can be prevented by inserting the needle into the vein on the first attempt; by avoiding multiple attempts at the same site; and by applying pressure on removal of the needle.
Timing of Collection
Therapeutic Drug MonitoringIf the blood is being collected to measure a drug level, Find out when the patient took the last dose of the drug in question and determine the best time to take the blood level.
- Trough levels - take the blood just prior to the next dose (do not withhold the dose while you wait for the result unless you're concerned that it's too high)
- Peak levels - find out the optimal time to take the sample depending on the drug
- Area under the curve (e.g. gentamicin) - consult guidelines and take blood at two set points following drug administration
- For glucose levels or glucose tolerance testing, the patient should fast for 8-10 hours prior to blood collection.
- For triglyceride testing (fasting lipids), the patient should fast for 10-12 hours prior to collection.
- Changing from a supine to an upright position can increase haematocrit, red blood cells, white cell count, calcium, thyroxine, AST, ALP, immunoglobulins, total protein, albumin, lipids, adrenaline, noradrenaline and renin levels.
- This is not usually sufficient to change a patient's results significantly, though wait for at least a minute after repositioning a patient before collecting blood.
- Emotional stress can cause a transiently elevated white cell count, cortisol and catecholamines. It can also result in hypercapnia in a blood gas sample, due to hyperventilation.
- Ensure that the patient is comfortable and have been resting for at least 15 minutes before collecting blood.
Prolonged Tourniquet Application
- Tourniquets are a very useful tool for identifying veins when taking blood or inserting a cannula. However, leaving a tourniquet on for too long can affect the interpretation of blood test results.
- Ideally the tourniquet should be left on for no longer than a minute at a time - if you are having difficulty finding a vein, then release the tourniquet, wait for blood flow to return, then reapply the tourniquet.
- Always remember to release the tourniquet when leaving a patient's bedside, as very prolonged tourniquet application can result in significant complications.
- The effects of prolonged tourniquet application include:
Effects on the Patient
- Nerve palsies - numbness, paraesthesias or weakness
- Limb ischaemia
Effects on Blood Results
- Haemoconcentration - water and certain solutes extravasate into the extracellular space, resulting in falsely elevated results
- Elevated lactate
- Elevated total protein, AST, lipids, cholesterol and iron
- Taking blood from the same limb that a heparin infusion is running into will falsely elevate the APTT. In order to avoid this, take the sample from a different limb, or turn off the infusion for 5-20 minutes prior to taking the sample.
Confounding of Blood Gas Testing
- Blood gases should be tested straight away, as ongoing cellular metabolism will occur while specimens are left without testing.
- Effects include reduced pO2, increased pCO2, reduced pH, reduced calcium, increased glucose and increased lactate.
- Cool the sample to 0-4 degrees using ice if it is unlikely to be processed within 15 minutes.
- The red blood cells in a sample may break down after sampling. This will affect the validity of potassium, LFT, amylase, creatine kinase (CK), folate, glucose, LDH and crossmatch results.
- The most common causes of sample haemolysis are:
- Vigorous mixing of the sample
- Drawing blood from a haematoma
- Drawing too quickly
- Using a needle that is too small
- Forcing the blood into a collection tube using a syringe
- A blood sample is liable to clot if it is left sitting for too long before placing it in the correct tubes, or if the blood is inadequately mixed within a tube. This may affect the validity of multiple results including the white cell count, red cell indices, platelet count and the coagulation profile.