Hemolyzed samples account for up to 70% of pre-analytical errors in veterinary clinical pathology. Proper venipuncture technique, appropriate needle gauge, and correct sample handling can virtually eliminate this avoidable source of diagnostic error. The Bloodwork OCR tool can help interpret results, but only when the sample is collected correctly.
Jugular vein: The preferred site for obtaining large blood volumes in both dogs and cats. The jugular provides the highest quality samples with the least hemolysis because of its large diameter. The patient is restrained in sitting or lateral position, with the head elevated and turned slightly away. The restrainer occludes the vein at the thoracic inlet while the phlebotomist identifies the vein in the jugular groove between the sternocephalicus and brachiocephalicus muscles.
Cephalic vein: Located on the dorsal surface of the forelimb, this is suitable for smaller volumes (1-3 mL) and is the most accessible site in fractious cats when jugular restraint is difficult. The restrainer holds off the vein at the elbow while extending the carpus.
Lateral saphenous vein (dogs): Runs across the lateral surface of the hock. Useful when forelimb access is compromised or for simultaneous catheter placement and blood draw. The patient is placed in lateral recumbency with the upper leg pulled forward.
Medial saphenous vein (cats): The feline equivalent, located on the medial surface of the hindlimb. Excellent for blood draws in cats that resist jugular restraint. The cat is placed in lateral recumbency or held upright with the leg extended.
Marginal ear vein (dogs): Used primarily for blood glucose monitoring or PCV/TS when only a few drops are needed. Not suitable for large-volume collection.
Needle gauge should match the vein size: 22-25 ga for cats and small dogs, 20-22 ga for medium dogs, and 18-20 ga for large dogs. Smaller needles reduce patient discomfort but generate higher shear forces that can hemolize red blood cells, particularly when combined with excessive aspiration pressure. Use a 3-6 mL syringe for routine draws; a 12 mL syringe for larger volumes. Butterfly catheters (23-25 ga) are useful for small, fragile veins or wiggly patients.
Vacuum tube systems (Vacutainer) are efficient for multiple-tube draws from the jugular vein. The standardized vacuum ensures consistent fill volumes. However, the strong vacuum can collapse small veins, making syringe-and-needle technique preferable for peripheral sites in small patients.
When collecting multiple tubes, the order of draw prevents cross-contamination of additives between tubes. The standard veterinary order of draw is:
| Order | Tube Color | Additive | Tests |
|---|---|---|---|
| 1st | Blue top | Sodium citrate | Coagulation (PT, PTT) |
| 2nd | Red top / SST (gold) | None / Clot activator + gel | Chemistry, serology |
| 3rd | Green top | Lithium heparin | Chemistry (stat) |
| 4th | Purple/lavender top | EDTA | CBC, blood film, crossmatch |
| 5th | Gray top | Sodium fluoride/oxalate | Glucose, lactate |
When transferring from a syringe, remove the needle first and gently expel blood into tubes. Never push blood through the needle—this causes hemolysis. Gently invert EDTA and citrate tubes 8-10 times to mix with the anticoagulant; do not shake vigorously.
The total circulating blood volume in dogs is approximately 80-90 mL/kg; in cats, approximately 60-70 mL/kg. A safe single blood draw should not exceed 1% of body weight (equivalent to approximately 10% of blood volume). For a 5 kg cat, this means a maximum of approximately 5 mL; for a 25 kg dog, approximately 25 mL. In anemic, hypovolemic, or debilitated patients, draw the absolute minimum volume required.
Hemolysis prevention: Use an appropriately sized needle (avoid excessively small gauges), apply gentle aspiration pressure (do not yank the plunger), transfer blood to tubes without the needle, avoid vigorous mixing, and process samples promptly. Hemolysis falsely elevates potassium, AST, LDH, and CK while interfering with bilirubin and lipase measurements.
Lipemia management: Lipemia (milky serum) is primarily caused by postprandial triglyceride elevation. Whenever possible, collect samples after a 12-hour fast. Lipemia interferes with spectrophotometric assays, falsely elevating bilirubin, hemoglobin, and total protein while potentially lowering electrolyte measurements. If lipemic samples are unavoidable, centrifuge at higher speeds and inform the laboratory.
Warning: Never collect blood from a limb receiving IV fluids. Sample dilution will cause falsely decreased PCV, total protein, glucose, and electrolyte values, potentially leading to inappropriate clinical decisions. If no other site is available, temporarily pause the infusion for at least 3 minutes and draw from a site distal to the catheter.
Once samples are collected and labeled, use the Bloodwork OCR tool to digitize and interpret results, or consult the Internal Medicine Specialist for differential diagnoses based on abnormal findings.
- Jugular is preferred for large volumes — produces the highest quality, least hemolyzed samples.
- Follow the order of draw — citrate first, then serum, heparin, EDTA, and fluoride to prevent additive contamination.
- Safe draw limit is 1% of body weight — approximately 10% of total blood volume at one collection.
- Remove the needle before transferring — pushing blood through a needle is the leading cause of in-clinic hemolysis.
- Fast patients 12 hours when possible — lipemia interferes with numerous chemistry assays.