Automated CBC (impedance and flow cytometry)

Principle

• Impedance (Coulter principle): cells suspended in an electrolyte pass one at a time through a small aperture between two electrodes; each cell briefly raises electrical resistance, producing a pulse.
• The number of pulses gives the cell count and pulse height is proportional to cell volume.
• RBCs and platelets are counted in the same channel and separated by size thresholds (platelets roughly 2–20 fL; RBCs above about 36 fL).
• Flow cytometry: cells are aligned by hydrodynamic focusing and pass a laser beam; forward scatter reflects size, side scatter reflects internal complexity/granularity, and fluorescent nucleic acid dyes help separate WBC types, reticulocytes and NRBCs.
• Haemoglobin is measured photometrically after lysis (cyanide-free sodium lauryl sulfate or cyanmethaemoglobin chemistry).

Specimen

• Venous blood in K2EDTA, correct fill, mixed by gentle inversion; free of clots.
• Analyse ideally within a few hours; most parameters are acceptable up to about 24 h at room temperature, but morphology and some parameters (e.g. MCV) drift with storage.

Results

• Measured directly: RBC, WBC, platelet counts, Hb, MCV.
• Calculated: Hct (RBC × MCV), MCH, MCHC; RDW from the RBC size histogram.
• Flags (e.g. blasts, atypical lymphocytes, platelet clumps, NRBCs) trigger a blood film review.

Quality control and pitfalls

• Run commercial controls (usually three levels) at defined intervals; calibrate per manufacturer; apply Westgard rules and delta checks.
• Cold agglutinins: falsely low RBC, high MCV and very high MCHC; warm sample to 37 °C and re-run.
• Lipaemia, very high WBC or paraproteins: falsely high Hb, MCH and MCHC.
• Platelet clumping or satellitism (EDTA-dependent): falsely low platelets; recollect in citrate.
• Microcytes, RBC fragments or WBC fragments may be counted as platelets; giant platelets may be missed or counted as RBCs.
• NRBCs may be counted as WBCs on some analysers; correct the WBC if needed.

Clinical use

• First-line screen for anaemia, infection, haematological malignancy, bleeding risk and treatment monitoring.