Laboratory Procedures
Laboratory procedures tests sample collection and handling, anticoagulant selection, hematology, chemistry, urinalysis, cytology, and quality control. The exam emphasizes preanalytical errors because most inaccurate results begin before the analyzer runs.
Core rules and concepts
- Follow the order of draw for vacutainer tubes: blood cultures, then citrate (blue), serum (red or gold), heparin (green), EDTA (lavender), and fluoride-oxalate (gray). Fill citrate tubes to the marked line because underfilling alters coagulation results.
- EDTA preserves cell morphology and is preferred for CBCs; heparin can be used for some chemistries but distorts cell staining. Serum tubes allow clotting before centrifugation.
- Make blood smears promptly from fresh EDTA blood, ideally within an hour. Evaluate feathered edge for platelet clumping and red cell morphology.
- Urinalysis includes physical properties, dipstick chemistry, and sediment examination. Refractometer readings estimate specific gravity; dipstick protein can be falsely elevated in alkaline urine.
- Cytology samples should be thin, air-dried, and fixed. Fecal flotation concentrates parasite eggs; direct smears are less sensitive.
Worked example
A serum sample shows marked hyperkalemia and hypocalcemia with no clinical match. The likely cause is EDTA contamination from a lavender tube drawn or decanted into the serum tube. EDTA binds calcium and releases potassium, producing a spurious pattern. Recollect the sample in the correct tube and document the rejection.
Traps that cost marks
- Underfilling citrate tubes, which falsely prolongs coagulation times.
- Refrigerating EDTA blood for a CBC or delaying smears, which distorts cells.
- Reading hemolysis or lipemia as true chemistry abnormalities without noting the interference.
- Reporting a urinalysis without examining sediment.