Monday 15 May, 2006

Blood collection from intravenous cannula

Posted in Emergency Dept., General Practice, Michael Tam, Procedures, Wards at 16:12 by Michael Tam

Index: Alternatives to venepuncture :: Printer friendly

Original article by: Michael Tam

Bloods can almost always be collected from a new cannula as long as you remember and can often be collected from an old one.

Blood at the time of cannula insertion


  • Your usual equipment for cannula insertion
  • 2 x 10 mL syringe (aspirate for pathology and flush)
  • 10 mL of sterile 0.9% NaCl (normal saline) for injection
  • 21 gauge needle (inject blood into pathology tubes)
  • pathology tubes

Blood collection is almost always possible from a newly sited intravenous cannula so don’t ruin another vein by poking the patient a second time.

A “Vacutainer” system with vacuumed tubes should never be used with a cannula. Rather learn the art of using a syringe.

  1. Tighten the tourniquet again.
  2. Attach a syringe to the cannula hub directly, or to a short extension line and gently aspirate back.
    • The advantage with using a syringe is that you can “feel” the flow of the blood back and adjust your pressure accordingly.
    • Too much suction and the vein collapses stopping blood flow.
  3. The position and angle of the cannula into the skin may have to be adjusted to get a good flow.
  4. After the specimen has been collected, release the tourniquet.
  5. Flush the cannula with 10 mL of 0.9% NaCl solution.
  6. Finish taping the cannula and fixing it to the skin as per normal.

Blood collection from an old cannula


  • 1 x 3 mL syringe (initial flush)
  • 3 x 10 mL syringes (first aspirate for discard, second aspirate for pathology, end flush)
  • tourniquet
  • 10 mL of sterile 0.9% NaCl (normal saline) for injection
  • 21 gauge needle (for injection of blood into pathology tubes)
  • pathology tubes

Taking blood from an “old” cannula, especially one sited in a small vein is often less successful, especially if the blood has been diverted from the vein distal to the cannula. Depending on the vein, there is probably only around a 50% chance of success. Nevertheless, it is worthwhile trying in someone with poor veins as if you are careful, you won’t “ruin” the cannula and the only thing you lose potentially is time.

  1. Stop any infusion into the cannula for at least 10 minutes.
  2. Give a 1-2 mL flush of the cannula with sterile saline.
  3. Apply the tourniquet.
  4. With a syringe attached to the hub of the cannula or to a short extension line, gently aspirate back.
    • Slightly adjusting the position of the cannula on the skin may facilitate flow.
  5. If blood can be aspirated then discard the first 10 mL of aspirate as it is contaminated with infusion fluid.
  6. With a new 10 mL syringe, aspirate the volume of blood required for your specimen.
  7. Once enough blood has been collected, release the tourniquet.
  8. Flush with 10 mL of sterile saline.


  • I recommend using 10 mL syringes for aspiration of blood as it gives you a good balance between size, and pressure control.
  • Switching between multiple syringes to collect the adequate volume of blood necessary if fiddly. It is uncommon that you actually need more than 10 mL of blood for most tests. If you do have many tests, use paediatric tubes or speak to the pathology laboratory scientists and technicians to let them know that you want tests done despite less than the usual amount of blood.
  • You cannot use this method to collect blood for an aPTT or coagulation studies if heparin has been run through the cannula as even a trace amount of heparin in the cannula will substantially change the result.

Updated: Michael Tam (19 June 2006)

Please read the disclaimer


  1. Sharmishtha Ghangrekar said,

    I have heard time and time again that I SHOULD NEVER take blood from a cannula… likely contamination despite discarding etc…

    The reasons I have been given in the past have not always been satisfactory but it has been advice from trusted seniors….

    What reasons do you see for not using it/using it and why…?


    • Hanmant said,

      if you want to collect blood from canula collect blood when
      you are instering canula for first time.
      *If canula is old please don’t collect blood from old canula, as it might have used for anticoagulants, antibiotics or other drugs or fluids and it may contaminate the blood sample which can affect test results.
      *Also if blood is not collected properly in sample tubes, there are chances that you are smearing the outside of the container and exposing the handlers to blood borne infections.
      *Also if sample is collected immediately after any infusion, it can dilute the blood and gives eratic results.

  2. james mudd said,

    Nurses ALWAYS draw blood for coag labs through heparin-flushed lines at our hospital. I have mentioned that this is not a good idea, but I am always ignored. How much of a problem is this, for example, if the line has been flushed with saline several times after the heparin flush? Will the PTT still be prolonged?

  3. Michael Tam said,

    I personally think that it is a problem. There are two issues. Firstly, if there is any contaminant heparin, it will substantially change the aPTT. If you look closely at a cannula, there is often a small area immediately adjacent to the injection site that has very poor flow. You can tell this, for example, during a blood transfusion where that area of the cannula will continue to have a tiny bleb of saline. At other times, there will be a small air bubble that never gets flushed away. It is conceivable in a heparin locked cannula that there will remain a small quantity of heparin in that part of the cannula despite flushing.

    Secondly, if the peripheral vein has relatively poor flow, there may be an issue with contamination with saline in the aspirate.

    At the end of the day you can never be sure of the accuracy of the test. Unlike a FBC or UEC where the effect from saline contamination is usually obvious, it is not obvious in coagulation studies.

  4. Rob said,

    Why does an infusion need to be ceased for 10minutes? Any fluid will surely enter circulation in a few seconds.

    • Michael Tam said,

      Simply because there can be very low flow of blood in a vein that has a PIVC inserted (in the order of millilitres per minute). You need to wait for the infusion fluid to be completely flushed clear of the peripheral vein.

  5. Anjali Salvi said,

    Can this method be used for ogtt studies or for collection of blood at various time points for glucose and insulin estimation,wut other precautions need to be taken.Please advise.

    • Hanmant Jadhav, Hyderabad said,

      Dear Anjali

      If you want to use canula for single purpose ( i.e. to collect blood sample only), I am sure you can use it, but take care that during collection of blood you are taking aseptic precautions to avoid contamination.

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