Specimen Collection for In-House and Outside Laboratory Evaluation
Key Takeaways
- Canine venipuncture sites include the jugular, cephalic, lateral saphenous, and lateral thoracic veins; feline sites include the jugular, medial saphenous, and femoral veins.
- Lavender-top (EDTA) tubes are for hematology, red-top or serum-separator tubes for chemistry and serology, blue-top (citrate) tubes for coagulation, and green-top (heparin) tubes for blood gas and electrolyte analysis.
- Proper labeling with patient ID, date, time, and collector initials must occur immediately at cage-side before leaving the patient to prevent specimen mix-ups.
- Chain of custody documentation tracks every handler from collection through in-house analysis or reference lab shipping, including temperature requirements and transport media.
- Urine is collected by cystocentesis when sterile samples are needed for culture, by catheterization when quantitative culture is required, and by free catch only when sterility is not a concern.
Venipuncture Sites by Species
Selecting the correct venipuncture site depends on species, temperament, vein accessibility, and the volume of blood needed. The veterinary technician must be proficient at all standard sites and choose the least stressful option that still yields a diagnostic sample.
Canine Venipuncture Sites
| Site | Location | Best For | Notes |
|---|---|---|---|
| Jugular vein | Ventral neck, bilateral to trachea | Large volume draws, emergency collection | Most common for dogs >5 kg; requires head extension and lateral or dorsal recumbency |
| Cephalic vein | Cranial forelimb, along radius | Small-to-moderate volumes, IV catheter placement | Most commonly used for awake dogs; requires minimal restraint |
| Lateral saphenous vein | Caudolateral hindlimb | Small volumes, cats and small dogs | Good alternative when cephalic is inaccessible; fragile in large dogs |
| Lateral thoracic vein | Lateral chest wall, caudal to elbow | Moderate volumes in recumbent or anesthetized dogs | Useful for donor dogs or surgery patients |
Feline Venipuncture Sites
| Site | Location | Best For | Notes |
|---|---|---|---|
| Jugular vein | Ventral neck | Large volume draws, most cats | Preferred for cats; "head lock" or lateral recumbency with forelimbs pulled caudally |
| Medial saphenous vein | Medial hindlimb | Small volumes | Commonly accessible in cats; less risky than femoral for hematoma |
| Femoral vein | Medial thigh | Small-to-moderate volumes | High-pressure vein; apply firm post-draw pressure to prevent hematoma |
The cephalic vein can also be used in cats but is less commonly accessed than the jugular or medial saphenous due to the ease of feline jugular restraint with a towel wrap.
Blood Tube Selection
The tube color determines the additive and the tests that can be run from that sample. Drawing into the wrong tube invalidates results.
- Lavender-top (EDTA): Hematology (CBC, manual differential). EDTA chelates calcium, preventing clotting. Also used for blood smears and some PCR/serology tests. Overfilling dilutes plasma and falsely lowers values.
- Red-top (no additive) or tiger-top (serum separator): Chemistry panels, serology, endocrine testing (T4, cortisol). Allow to clot 15–30 minutes before centrifuging. Sseparator gel in tiger tubes prevents cellular contamination of serum during transport.
- Blue-top (sodium citrate): Coagulation panels (PT, aPTT, fibrinogen). Must fill to exact fill line (usually 90% full) — underfilling falsely prolongs clotting times due to excess citrate relative to plasma.
- Green-top (heparin): Blood gas analysis, electrolytes (especially ionized calcium), and STAT chemistry. Heparin inhibits thrombin; not suitable for coagulation testing.
- Grey-top (fluoride/oxalate): Glucose preservation (inhibits glycolysis). Used when glucose cannot be run immediately.
Order of draw: When multiple tubes are filled from one syringe, draw into the citrate (blue) tube first to minimize tissue-factor activation, then EDTA (lavender), then serum tubes (red/tiger), then heparin (green). This prevents cross-contamination of additives.
Labeling and Chain of Custody
Every specimen must be labeled at cage-side, before leaving the patient, with:
- Patient name and/or ID number
- Date and time of collection
- Collector initials
- Sample type and tube type (if ambiguous)
Labels applied after walking to the lab bench risk misidentification — the single most common source of pre-analytical laboratory error. If a tube is unlabeled and the collector cannot identify it with certainty, the sample must be discarded and recollected.
Chain of custody documentation records every transfer of the specimen: who collected it, who received it, when it was accessioned into in-house analysis, or when and how it was shipped to a reference lab. For legal cases (suspected poisoning, animal cruelty investigations), this documentation may be subpoenaed and must be unbroken.
Shipping to Reference Laboratories
Reference labs require specimens packaged per IATA shipping guidelines:
- Temperature control: Refrigerated samples ship with cold packs (chemistry, hematology, urine). Frozen samples ship on dry ice (certain endocrine tests, drug levels). Never freeze whole blood — hemolysis destroys RBCs and invalidates chemistry values.
- Leak-proof secondary containment: Primary tubes go inside a sealed plastic bag with absorbent material, then inside a rigid shipping container.
- Submission forms: Complete lab-specific requisition forms with patient signalment, tests requested, and relevant history. Mismatched forms and samples cause accessioning delays.
- Turnaround awareness: Some analytes degrade rapidly — bilirubin is light-sensitive and should be protected from direct light; glucose drops ~10% per hour in uncentrifuged whole blood at room temperature.
Other Specimen Types
- Urine: Cystocentesis (needle through abdominal wall into bladder) yields the most diagnostically useful, sterile sample for culture and sediment examination. Catheterization is used for quantitative culture or when cystocentesis is contraindicated. Free catch is acceptable for routine urinalysis but not culture.
- Fecal: Fresh (<2 hours old) or refrigerated in airtight container. Floatation, direct smear, and antigen tests detect parasites. Never freeze feces — protozoal cysts rupture.
- Cytology: Fine-needle aspiration (FNA) uses a 22–25g needle with or without suction. Smear immediately, air-dry for Diff-Quik or fix in methanol for Wright-Giemsa. Submit unstained slides plus any remaining sample in formalin for histopath if cellularity is low.
- Microbiology: Swabs in transport media (Amies, Stuart) for aerobic/anaerobic culture. Tissue samples in sterile red-top tubes without formalin. Fungal cultures require Sabouraud dextrose agar transport tubes.
The technician's judgment at cage-side — correct site, correct tube, correct labeling, and correct storage — determines whether downstream laboratory results are diagnostic or discarded.
A dog needs a CBC and a chemistry panel. Which two tubes should you draw, and in what order?
A cat presents for a suspected urinary tract infection and the veterinarian requests a urine culture. What is the preferred collection method?
Which venipuncture site is most commonly used for large-volume blood collection in an awake dog weighing 15 kg?