The indications for blood sampling in small animal practice are varied. Examples of patients from whom a blood sample may be required to assist in their diagnosis and ongoing treatment monitoring include patients with medical conditions, surgical patients and emergency patients. Blood samples are also indicated in healthy animals as part of routine health checks and to obtain baseline values, for example in senior patients and pre-anaesthetic patients. The decision about which blood analyses to perform is the remit of the veterinary surgeon (VS), however it is essential that student and registered veterinary nurses (SVN and RVN) are suitably trained to undertake venepuncture; it is a task they can undertake for the VS to assist in the prompt assessment, diagnosis and treatment monitoring of all patients. A range of analyses can be performed using a blood sample including haematology, manual packed cell volume (PCV) measurements, blood smear examination, biochemistry including electrolyte analysis, blood gas analysis and various laboratory assays, but a good quality blood sample is crucial to obtaining accurate results.
Obtaining blood samples
Blood samples are commonly obtained from the jugular, cephalic or lateral saphenous veins; the medial saphenous vein can also be used, and in general the use of the jugular vein is recommended. The RVN or SVN should be proficient in the technique for optimal sample procurement to prevent problems with the sample that may impact accurate analyses. There are models/manikins and fake vein boards (Figure 1) that can be purchased for practising blood sample techniques and intravenous (IV) catheter placement and, when the VS requests a blood sample for analysis, it may be wise for novice phlebotomists to practise on patients when they are under general anaesthetic because of their lack of movement.
The equipment that should be prepared for obtaining a blood sample includes (Figure 2):
- Restraint aids as required
- Correct blood tubes for the tests the VS has requested (see Table 1)
- Microscope slides and spreaders so fresh blood smears can be prepared for analysis
- A sterile syringe capable of collecting the volume of blood needed to fill the blood tubes to the fill line, and spares
- A 21G sterile needle and spares. Aseptically assemble the syringe and needle so the hubs of the syringe and needle do not become contaminated; this can be achieved without needing to don gloves. It is recommended you avoid ‘popping’ needles and syringes out of their packaging to reduce the risk of contamination
- Ensure the needle is mounted bevel-up and the syringe's plunger is pre-loosened
- Antiseptic skin preparation solutions/swab(s)
- Hand wash +/− gloves
- Sharps bin
- Swab for post-venepuncture pressure
- Bandage materials in case prolonged pressure is required over the venepuncture site
- Good light source
- Clean, functional clippers — ideally quiet ones, especially for cats.
Table 1. Blood tube anticoagulants
| Cap colour | Anticoagulant | Comments |
|---|---|---|
| Orange | Lithium heparin | Yields plasma for biochemistry analyses |
| White | No anticoagulant — plain/serum tube | Yields serum for biochemistry analyses. Allow to clot, centrifuge, decant serum into a new plain tube |
| Pink | EDTA | For haematological analyses |
| Green | Sodium citrate | For coagulation tests |
| Brown | No anticoagulant. Has a gel barrier (serum tube) | Yields serum for biochemistry analyses. Allow to clot then centrifuge. Gel clot sits between the clot and the serum to protect the serum |
Repeat blood sampling and testing may be requested by the VS at various intervals to compare against the baseline results from admittance, in order to identify improvement or deterioration of the patient's condition.
The process for optimal blood sample procurement is given in Table 2.
Table 2. A 20-step process for optimal blood sample procurement, including commentary about potential pitfalls throughout the procedure
| Step | Activity | Comments/reminders |
|---|---|---|
| 1 | Wash hands | The operator's and restrainer's hands should be washed prior to starting the procedure. Don disposable gloves |
| 2 | Ensure all equipment is prepared for sampling | Once the patient is being restrained you must have all of the equipment you need at hand, otherwise the patient will be restrained for longer periods than necessary and this may increase or exacerbate stress, resulting in patient movement |
| 3 | Check the assembled 21G needle and syringe to ensure the needle is mounted bevel-up | Anything smaller than 21G risks damaging the red blood cells during sample procurement. Bevel-up needles ensure the sharpest tip of the needle pierces through the skin and into the vein first (Figure 3).Please note: if it is suspected the patient may have a clotting disorder it would be prudent to use a 23G needle for blood sampling, despite the drawbacks mentioned above. The operator just needs to be even more mindful of red blood cell damage in these cases and be very gentle |
| 4 | Double check the plunger is pre-loosened | If this is not done then slightly more force is required to loosen the plunger when the needle is already seated in the vein. This results in a slightly less controlled first draw of blood, applying too much suction pressure, which can damage the red blood cells, resulting in haemolysis |
| 5 | Request patient restraint according to the vein selected | Comfort of the patient, the operator and the person restraining must be considered. For many patients it is often easiest to place them on an examination table. The person restraining must be proficient in restraint of that species and knowledgeable of the location of the vein requested. The operator must guide the person restraining if any adjustments are required to the positioning |
| 6 | Clip a patch of fur away over the vein (Figure 4). Remove all loose fur and discard appropriately | The operator can palpate the general area where the vein should lie if they wish to assess its potential location before clipping. This is often wise to do in patients where their anatomical composition poses challenges, for example in breeds with very short and/or slightly malformed legs.The clip does not need to be excessive — just large enough to allow fur-free access to the vein. It is important to remove the loose fur following clipping to avoid it inadvertently causing contamination |
| 7 | Ask for the vein to be raised by the assistant (cephalic or saphenous (Figure 4) or raise it yourself (jugular) to double check the fur clip is in the correct place | Once occluded, the vein should engorge with blood that is trying to return to the heart — this makes it ‘rise’. Remember in hypovolaemic patients this rise may not be very prominent at all, so the operator's knowledge of vascular anatomy is vital for successful sample procurement |
| 8 | Use the antiseptic solutions to clean the skin over the vein | The operator may not have been wearing gloves up to this point, which is acceptable. It is good practice to wear gloves from this point forwards. Skin preparation is essential to prevent bacteria from the skin entering the vessel. It is recommended skin is decontaminated by wiping the skin with 2% chlorhexidine gluconate in 70% alcohol and allowing this to dry before inserting the needle (National Institute for Health and Care Excellence, 2014; Invicta Animal Health, 2020) |
| 9 | Ask for the vein to be raised (if the assistant has stopped) or raise it yourself (jugular) | |
| 10 | For cephalic and saphenous veins, use your other hand to hold the limb and use your thumb alongside the vein to stabilise it | At this point the operator should not touch the cleaned area again, just to double check where the vessel is. If they do, the insertion site must be cleaned again before inserting the needle |
| 11 | Insert the needle, bevel-up, through the skin and into the vessel. A fairly shallow angle is sufficient but will be dictated by the size of syringe you are using | 5 ml syringes and larger tend to have wide barrels, which means it is impossible to get the whole needle and syringe unit fairly flat to the patient's limb. It is possible to purchase syringes where the hub for needle attachment is to one side of the barrel as opposed to being central, which will facilitate more shallow angles during sample procurement (Figure 5) |
| 12 | Withdraw the plunger gently and steadily until you have the correct volume of blood in your syringe | Do not apply excessive pressure — go gently, drawing back bit-by-bit, allowing the syringe to fill in a slow and controlled manner |
| 13 | Ask the assistant to stop raising the vein (cephalic/saphenous) and apply pressure over where the needle has been removed using a swab | Once the correct volume of blood has been obtained the restrainer can release the pressure on the vein and the needle can be withdrawn by the operator. Continual raising of the vein once the needle has been removed will promote blood leakage from the vessel through the hole created due to the internal vessel pressure being created |
| 14 | Remove the needle carefully and discard into the sharps bin | The operator should do this carefully with their fingers grasping the hub of the needle, or artery forceps can be used instead of fingers for improved safety. Sharps bins do have areas on their lids designed for needle removal, but using these risks contamination the hub of the syringe as the needle slides off |
| 15 | Decant the blood into the blood tubes carefully (do not squirt it forcefully — do it drop-by-drop) until the fill line is reached | Slightly faster ejection/steadier flow of the blood from the syringe into the tube is acceptable initially, but as the fill line is approached it is wise to slow down and decant the blood drop-by-drop to avoid overfilling the tube.The blood should not be decanted into the tubes through the needle as this is an unnecessary risk to take in relation to potential red blood cell damage.Take care to not touch the hub of the syringe to the inside surface of any of the blood collection tubes used; this may contaminate the hub of the syringe with anticoagulant and affect results |
| 16 | Discard the contaminated syringe into the appropriate waste bin | |
| 17 | Recap the blood tubes and gently invert (do not shake) at least 5 to 10 times | Inverting this many times, in addition to filling the tubes to the fill line, ensures correct mixture of the blood with the anticoagulant in the blood tube. Too little blood results in an increased anticoagulant to blood ratio, and too much blood results in a decreased anticoagulant to blood ratio. Incorrect mixtures can potentially affect the tests run using the blood, or could result in clot formation, wastage of that sample and having to take more of the patient's blood. This is not acceptable |
| 18 | Double check the tubes are labelled and send to the laboratory for analysis | It is always wise at this point to visually check the quality of the sample in the tube — is there any evidence of haemolysis or clots forming? If there are, you will need to obtain a fresh blood sampleIt is also advised to check at this point whether any further preparations are required, for example some samples need to be spun prior to sending to an external laboratory. You should refer to the information provided to you by the external laboratory your practice uses |
| 19 | Check the venepuncture site for signs of bleeding or haematoma formation and take appropriate action | Once the blood has been safely decanted, mixed and sent for analysis the operator should return to the patient, examine the venepuncture site for bleeding or haematoma formation. A pressure dressing may be required if there is evidence of either, however this must be written on the patient's hospitalisation sheet to indicate when it needs to be removed, and this information should be communicated to others caring for that patient.If a patient is suspected of having a clotting disorder then the blood must be obtained from either the cephalic or saphenous vein, NOT the jugular. It needs to be a place where a pressure dressing can safely be applied, which does not include the neck |
| 20 | Complete the patient's records to indicate that a blood sample has been obtained | It is very important to record when samples have been taken and tests run so they are not repeated unnecessarily, and the veterinary surgeon can then plan timing intervals for repeat sampling accurately |
(Cooper et al, 2011; Aldridge and O'Dwyer, 2013; Davis et al, 2013; Ackerman, 2016; Duke-Novakovski et al, 2016
Conclusion
RVNs and SVNs must develop their skills and competency regarding venepuncture and be mindful of the potential pitfalls when procuring a diagnostic sample. As outlined throughout this article, there are many points at which problems can occur when procuring a blood sample that can adversely affect its diagnostic quality. All veterinary practices should provide the training required to assist in the development of these skills as blood sampling and analysis is frequently required, is essential in some cases for the VS to diagnose conditions and make treatment plans, and the veterinary nursing team are more than capable of supporting their VSs with their case management in this respect.
KEY POINTS
- Blood sampling and analysis provides a wealth of information for the veterinary surgeon about a patient's health when diagnosing conditions and treatment planning.
- Safe and effective procurement of blood samples is achievable with appropriate training of the student veterinary nurse (SVN) and registered veterinary nurse (RVN).
- There are numerous veins available for obtaining a blood sample, and the SVN or RVN should develop their skills in utilising all vessels for this purpose.
- SVNs and RVNs competent in venepuncture can save the veterinary surgeon time during case management.