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PA13.2-4 | Performing Common Haematological Tests: Hb, RBC/WBC, DLC — Part 2

Calibration, Quality Control, and the Delta Check

Daily Analyzer Calibration

Automated haematology analyzers require:
1. Morning startup QC: Run 3 levels of commercially prepared whole-blood control material (low, normal, high) before the first patient sample.
2. Acceptance criteria: Each parameter must fall within ±2 SD of the control's target range (Westgard rules). A single parameter outside ±3 SD = instrument failure — do NOT report any results until the cause is identified and corrected.
3. Levey-Jennings chart: Plot daily QC values for each parameter on a control chart. Trending (6 consecutive points on one side of the mean) = systematic error (dirty aperture, reagent lot issue) before the value exits the ±2 SD range. Catching trends early prevents a batch of bad results.
4. New reagent lots: Run the old and new lots in parallel on 20 samples before switching — lot-to-lot variation in lyse reagent can shift WBC differential percentages by 3–5%.

Laboratory Quality Control Systems in Hematology

The Delta Check

For any patient with a previous result on file, the analyzer system computes the delta (change from last result). If the delta exceeds a preset limit (e.g., Hb drops > 3 g/dL without a documented clinical event, or creatinine doubles overnight), the system flags for manual review.

The delta check catches:
- Sample mix-ups (wrong tube labeled with wrong patient name)
- Pre-analytical errors (haemolysis, clotted sample diluting the previous draw)
- Genuine acute events (GI bleed overnight — Hb delta will flag, prompting clinician call)

A delta flag does NOT always mean an error — it means a human must look before reporting.

Common Errors and the 'Smear Review' Rule

Pre-Analytical Errors and Their Effects

Common mistakes callout:

Error	What you see on report	Clue	Corrective action
Clotted EDTA sample	Low platelet, distorted cell morphology, spuriously low WBC (cells in clot)	Fibrin strand visible in tube, platelet count < 50,000 in otherwise well patient	Recollect
Prolonged storage at RT (> 4 h)	MCV falsely high (RBC swell), WBC counts less affected, platelet counts drop	Old sample time stamp; RBC histogram shifted right	Recollect
Haemolysis (fragile veins, forceful aspiration)	Falsely elevated K⁺; RBC count low; Hb measured from haemolysate may be accurate but MCV/MCH/MCHC unreliable	Pink supernatant in tube, 'H' flag on analyzer	Note haemolysis, recollect if clinical decision depends on it
Wrong dilution (pipette error)	All counts uniformly high or low	All parameters shift proportionally; Hb and RBC inconsistent with calculated MCHC	Repeat with fresh dilution

When Must You Review the Peripheral Smear?

Automated analyzers output flags — 'blasts?', 'atypical lymphocytes?', 'left shift?' — that require smear confirmation. Hard indications for smear review:
- WBC > 30,000 or < 3,000/µL
- Platelet < 100,000 or > 1,000,000/µL
- Morphology flags (blasts, giant platelets, RBC fragments)
- Any patient with malignancy, haemolytic disease, or splenomegaly
- Delta check flag without a clinical explanation
- Discrepancy between calculated MCHC (from analyzer) > 36 g/dL (physically impossible — indicates error or spherocytosis)

The 'automated count vs smear' rule (Golden Rule of haematology lab): A normal automated CBC in a well patient with no flags = report without smear. Any flag, any clinical suspicion, any delta alert = smear review before reporting.

Worked Clinical Example

Laboratory Quality Control Workflow: Pre-analytical to Post-analytical Steps

Scenario: EDTA tube, 4 mL drawn from a 5 mL tube. Automated result: Hb 9.5 g/dL, WBC 11,000/µL, Platelets 50,000/µL. Patient is a 35-year-old woman, no known bleeding history, referred for fatigue.

QC checks before reporting — step by step:

Step 1 — Pre-analytical inspection
- Is the tube fill adequate? 4 mL of a 5 mL EDTA tube = 80% fill. Acceptable (underfill <60% causes relative EDTA excess → cell shrinkage → falsely low MCV).
- Any fibrin strand or clot visible? If yes → recollect.
- Sample age? Collected > 4 hours ago → note, especially if MCV is high.
- Patient name and date of birth on tube? Matches request form? (pre-analytical ID check).

Step 2 — QC log
- Was this morning's QC control run? Pass? If QC failed → do NOT report until instrument re-calibrated.
- Check the platelet control value this morning — if borderline high or trending, flag.

Step 3 — Delta check
- Previous Hb on file? If yes, Hb drop of > 3 g/dL since last visit → flag for clinical correlation (GI bleed? haemolysis? timing?).
- Platelets 50,000/µL in a previously normal patient → large delta → trigger smear review.

Step 4 — Smear review (mandatory here)
- Platelet count 50,000 is below the 'hard indications' threshold → make and review smear.
- Look for: platelet clumps (EDTA pseudothrombocytopenia), fragmented RBCs (microangiopathic haemolysis), hypersegmented neutrophils (megaloblastic anaemia), target cells (iron deficiency, thalassaemia).

Step 5 — Interpretation
- If smear shows platelet clumps → recollect in citrate tube → repeat count (likely pseudothrombocytopenia).
- If smear shows genuine thrombocytopenia with anisocytosis + pencil cells → supports iron deficiency anaemia + possible thrombocytopenia — discuss with clinician, do not transfuse based on automated count alone.

Conclusion: In this scenario, Hb 9.5 is likely real (fits clinically), but platelet 50,000 must be verified before reporting. The workflow — inspect → QC → delta → smear → report — is what separates a safe laboratory from a dangerous one.