4 easy ways to obtain diagnostic quality blood samples
Collecting good quality blood samples is a fundamental skill for veterinary nurses.
It’s perhaps even more important for patients with medical disorders, because we perform so many different tests in these patients!
We may also be more likely to see things like icterus in these patients, or have our sampling limited by conditions such as thrombocytopenia or coagulopathies - so understanding how to collect good quality samples in these patients is vital.
In this post, I’m going to share my top tips and nursing considerations for taking diagnostic quality blood samples - so that you can ensure you get the most reliable results you can for your patients!
Don’t forget, if you want to dive deeper into what these samples tell us, I’m hosting a workshop all about how to interpret biochemistry results on February 8th! It’s exclusive to members of the Medical Nursing Academy - the new CPD community for veterinary nurses interested in medical nursing. If you want to know more, make sure you’re signed up to the academy waitlist and I’ll let you know when doors open on February 1st!
Preparation is key
There are lots of different considerations when it comes to collecting blood samples. Before we take our sample, we need to think about:
The tests we need to perform
The sample type needed (most tests require venous blood, unless you’re performing arterial blood gases - you can read more about that here)
The medications the patient is currently receiving
When the patient was last fed
The patient’s temperament
The patient’s disease process
The site you intend to use
How you intend to take your sample
The equipment you will need
Seems like a lot to think about, right? But considering all of these factors will really help you get good quality samples.
Some medications, for example, may affect certain results (e.g. T4 testing is affected by things like phenobarbital and furosemide administration) - so it’s always worth checking prior to take blood for these tests.
Certain tests will be harder to interpret if the patient has been fed recently (e.g. bile acids and ammonia) - so if these tests are particularly important for your patient, pulling a fasted sample is vital.
Patients with some diseases may not tolerate jugular blood sampling. For example, patients with coagulopathies or platelet disorders, or patients with increased intraocular or intracranial pressure (since jugular occlusion increases this further).
This means, if jugular sampling is contraindicated, we’ll need to use an alternative site. Many of the other vessels we use are smaller, and can take longer to collect samples from. It’s important we consider this, as it may increase the risk of clot formation or haemolysis during sampling.
Speaking of sampling sites, there are several we can use, including:
Jugular veins
Cephalic veins
Medial saphenous veins
Lateral saphenous veins
Metatarsal veins
+/- Ear veins
The exact one you choose will depend on your patient, their size/breed/species and individual temperament and disease process. But there are lots more than our ‘big 3’ - so don’t be afraid to think outside of the box if those veins have been used lots already!
Let’s talk equipment
Traditionally, blood samples have been taken using a hypodermic needle and syringe. Vacutainer collection systems are available, but not commonly used in small animals as larger samples are required to fill the vacutainers.
We can also use butterfly needles to collect our samples, or, in some cases, collect the sample via an intravenous catheter.
Needle size - does it matter?
When using a needle and syringe, the needle size should be one the patient can comfortably tolerate whilst not being small enough to cause damage to the cells. Recent studies have examined the role needle size plays in haematological parameters in cats and dogs, and found no clinically meaningful difference in clotting times or platelet numbers when using either a 22g or 25g needle (cats) or a 21g, 23g or 25g needle (dogs).
The syringe should be one only large enough for the required sample volume. Larger syringes can place excessive negative pressure on the vein, collapsing it, and/or damaging collected cells.
Blood tubes - which ones should we use, and how should we fill them?
Once the samples are collected (we’ll talk more about sample collection and patient considerations later in this post) we need to decant them into the appropriate blood tubes. A variety of tubes are used in practice, including:
Plain (white) tubes, used to collect serum samples for biochemistry and/or serology
Gel barrier (brown) tubes, used to collect serum samples as above
Heparin-anticoagulated (orange) tubes, used to collect plasma samples for biochemistry and certain infectious disease panels
Sodium citrate (green or mauve) tubes, used to collect whole blood samples for coagulation testing
EDTA (pink, lavender or red) tubes, used to collect whole blood for haematology and some PCR testing
Fluoride oxalate (yellow) tubes, used to measure glucose levels in whole blood *not in the UK? You may find these tube colours are different!
It’s really important to maintain the correct ratio of anticoagulant to blood, since excess anticoagulant can affect results, and inadequate anticoagulant levels can cause clots to form. Fill each tube to the line - and remember that many ‘1ml’ tubes are actually ‘1.3ml’ - meaning you’ll need to take extra blood from your patient!
The order of draw
There’s also a specific order these tubes should be filled in to ensure accurate results, called the order of draw. This is important since anticoagulants can cause changes to biochemistry parameters, and clots in haematology or coagulation samples can significantly impact results.
The order is:
Blood culture bottles (if applicable)
Citrate tubes
Plain tubes
Gel barrier tubes
Heparin tubes
EDTA tubes
Fluoride oxalate tubes
So, when collecting your sample, fill your coagulation tests first, then your biochemistry samples, then your haematology samples.
Once your tube is filled, invert gently several times to mix. Avoid shaking the samples, since this can cause haemolysis.
Patient-friendly sampling considerations
A big part of getting an accurate sample is keeping your patient as happy and settled as possible before and during sampling.
A calm patient is less likely to have stress-associated changes to their results (e.g. a stress leukogram on haematology, or hyperglycaemia in cats).
We also want to utilise minimal restraint wherever possible - as well as being much nicer for our patients, we’re likely to see changes to things like CK (creatine kinase) associated with difficult sampling / excessive restraint.
If you have time (if your sample is non-urgent), utilise topical local anaesthetic creams, or topical cold-application sprays if you need to take your sample urgently. Balance needle size with the impact on your results and the tolerance of your patient, and select a sample site that works for your patient and your test - for example, the saphenous vein can be a nice site in nervous patients, as you’re not right by their head!
We’ve talked prep… now let’s collect our sample!
Here’s a step-by-step refresher for collecting a great quality blood sample:
Wash your hands +/- wear gloves if desired, or if your patient is a barrier nurse
Assemble your equipment. Make sure that your needle is bevel-up, and loosen your syringe plunger before use
Identify your sampling site and clip an appropriately-sized patch of fur (if not already clipped and EMLA’d)
Raise (or ask an assistant, depending on your sample site) and examine/palpate the vein
Clean the site with antimicrobial skin solution
Re-raise the vein (or ask your assistant to). Note that if you palpate the vein again here, you need to re-clean the site before inserting your needle!
Insert your needle, bevel-facing up, into the vessel. If you’re using the cephalic or saphenous veins, place your non-dominant thumb alongside the vessel to stabilise it
Gently withdraw the syringe plunger, minimising negative pressure (avoiding >0.5-1ml negative pressure) to minimise haemolysis
Continue until the required volume has been collected
Stop raising (or ask your assistant to), remove the needle, and ask your assistant to apply pressure
Decant your sample into your blood tubes according to the order of draw, filling each tube to the line
Rotate anticoagulated tubes to mix
Inspect your patient’s venepuncture site for bruising, haematoma formation or clipper rash. Treat any abnormalities you see accordingly
And that’s it - you’ve got your sample!
But it’s not always that easy, so I want to leave you with a couple of troubleshooting tips in case your patient doesn’t want to give you their blood!
Troubleshooting
If you’re collecting from a cephalic or saphenous vein, and the blood stops flowing, ask your assistant to de-raise and re-raise the vein intermittently. This can help prevent the vein collapsing and ensure blood continues to flow.
Consider using smaller syringes for smaller vessels. If you need to fill multiple syringes, you can use a butterfly catheter or an intravenous catheter instead, which allow syringes to be changed easily.
If your patient has really tricky veins, you can also use a 1ml syringe without the plunger attached to your IV catheter - this fills by capillary action, avoiding any negative pressure on the vein whatsoever.
So the next time your patient’s veins are being a little challenging, try these tips to get your sample more easily!
In conclusion, blood sampling is a fundamental skill for vet nurses. It can be seen as basic - but pulling a good quality sample and getting accurate results is a vital nursing consideration, especially for our medical patients!
When collecting your sample, think about the site, the individual patient, the test you’re running, and the equipment you’ll need. Prepare everything in advance, and make sure to keep that anticoagulant-to-blood ratio correct! When filling those tubes, remember the order of draw, and invert your anticoagulated tubes gently to mix them.
I’d love to know which of these tips you’re going to try for your next difficult sample - DM me on Instagram and let me know! And don’t forget, if you want to know more about interpreting results, make sure you’re ready and waiting for the academy doors to open on Feb 1st - as the Biochemistry 101 workshop will be held exclusively inside the academy!
References
Bloor, C. 2021. How to blood sample. The Veterinary Nurse [Online]. Available from: https://doi.org/10.12968/vetn.2021.12.1.34
Greenwell, CM. et al. 2014. Influence of needle gauge used for venipuncture on automated platelet count and coagulation profile in dogs. Australian Veterinary Journal, 82 (3), pp. 71-74.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians. 7th ed. Missouri: Elsevier.
Solbak, S. et al. 2019. Influence of needle gauge used for venipuncture on measures of haemostasis in cats. Journal of Feline Medicine and Surgery, 21 (2), pp. 143-147.
