Laboratory Procedures and Diagnostics
Laboratory procedures test sample selection, handling, and interpretation. The exam expects you to know which tube suits which test, how artifacts change results, and how to combine values such as PCV and total solids into a coherent interpretation.
Sample tubes and handling
- Lavender EDTA: CBC and blood smear; mix gently but do not refrigerate whole blood for routine counts.
- Serum separator or red-top: chemistry and serology; allow clotting, then centrifuge.
- Green heparin: blood gas and some chemistries; heparin can dilute small samples.
- Light-blue citrate: coagulation; fill to the marked line.
- Gray fluoride-oxalate: glucose.
Core measurements
- PCV from a microhematocrit tube; total solids by refractometer. Typical canine PCV is about 37-55% and feline 30-45%; interpret PCV and TS together.
- Fecal flotation concentrates ova; a direct smear is better for motile Giardia.
- Cytology smears should be thin, air-dried, and stained with a Romanowsky stain such as Diff-Quik.
Worked example
A dog has PCV 25% and total solids 8.0 g/dL. The low PCV indicates anemia; the high TS suggests dehydration or a protein-secreting process. Giving fluids will change the TS, so record both values before therapy and repeat after resuscitation. Do not call it just anemia without explaining the protein.
Traps that cost marks
Submitting EDTA blood for potassium because the tube was convenient (EDTA falsely raises potassium and lowers calcium); refrigerating a CBC sample; underfilling a citrate tube; reading a lipemic or hemolyzed sample as normal; and collecting a culture after antibiotics are started.