Clinical Biochemistry 1

Clinical Biochemistry – Purpose & Scope

• Supports diagnosis, monitoring, treatment decisions across NI’s $5$ hospital trusts (≈ $40$ million tests in $21/22$)
• Core lab (NHSCT) operates $24/7$; TAT: $1$ h urgent, $4$ h inpatient, $24$ h routine; >$3{,}000$ samples/day, $65+$ assays
• Point-of-Care Testing present in all hospitals

Specimen Pathway

• Pre-analytical: receipt → sorting (urgent / inpatient / routine) → accessioning in LIMS → centrifugation (serum/plasma; $7$ min) → automated aliquoting/tracking
• Analytical: automation lines (Roche Cobas c8000: ISE, photometry, ECL); specialist sections (Proteins, HbA1c, Osmolality, CSF)
• Post-analytical: instrument → middleware (Infinity) → LIMS (WinPath) → EPR (Encompass); technical validation applied before release

Major Analytical Techniques

• Potentiometry (ISE)
  • Ions $Na^+$, $K^+$, $Cl^-$; indirect (dilution $1:31$) measurement
  • Uses adapted Nernst equation (compares EMF of sample vs. internal standard)
  • Electrolyte exclusion effect: high lipids/protein ↓ liquid phase → falsely low results

• Photometry / Spectrophotometry
  • Beer–Lambert law: $A \propto c$ (absorbance proportional to concentration)
  • Endpoint & rate assays; bichromatic reading reduces interference
  • Example: Mg²⁺ + xylidyl blue (purple) measured at $600\,\text{nm}$

• Electrochemiluminescence (ECL) Immunoassay
  • Voltage-triggered Ru/TPA reaction produces light; intensity ∝ analyte
  • Biotin–streptavidin capture on magnetic beads
  • Sandwich (e.g. TSH) → direct; Competitive (e.g. FT4) → inverse relation

• Additional methods
  • Freezing-point depression for osmolality (formula: $2\,[Na] + \text{urea} + \text{glucose}$; gap = measured – calculated)
  • Capillary/Gel electrophoresis: serum proteins, HbA1c
  • CSF xanthochromia: spectral scan for bilirubin

Quality Management

• Internal QC (IQC)
  • Third-party controls (low/normal/high) run at start-up & throughout day
  • Acceptable if within $\pm 2\,SD$ of target; visualised on Levy-Jennings; Westgard rules flag shift/trend, imprecision, bias
• External QA (EQA)
  • Blind samples from schemes (e.g. NEQAS) assess accuracy, bias, consistency; retrospective traffic-light reports
• Calibration
  • Sets analyser–concentration relationship (typically $2$-point: water $+$ standard)

Error Sources & Detection

• Biological: age, sex, posture, diet, drugs, circadian timing
• Pre-analytical: mis-labelling, wrong tube, IV contamination, delayed transport/storage
• Analytical: mechanical faults, reagent issues, calibration drift, method variation (seen on IQC/EQA)
• Post-analytical: transcription/IT errors; mitigated by middleware rules, delta checks
• Delta check: current vs. previous result; flags change > preset %
• Clinical & phone limits: middleware blocks / prompts urgent communication when outside defined thresholds

Key Concepts / Definitions

• Reference range: $\mu \pm 2\,SD$ of healthy population (covers $95\%$)
• Accuracy: closeness to true value; Precision: repeatability
• Osmolar gap: presence of unmeasured osmoles (ethanol, EG)
• Serum indices (H, I, L): detect haemolysis, icterus, lipaemia; middleware suppresses affected results

Automation Benefits

• High throughput, consistent quality, reduced TAT, minimized biohazard exposure, integral QC monitoring

Clinical Relevance

• Correct results critical: errors → misdiagnosis, treatment delay or harm
• Some tests diagnostic alone (BNP, PSA), others need context (e.g. hyponatraemia work-up)