Blood Gas Analysis
Arterial blood gas (ABG) analysis measures pH, partial pressure of oxygen (PaO₂), partial pressure of carbon dioxide (PaCO₂), bicarbonate (HCO₃⁻), and oxygen saturation (SaO₂). It is essential for assessing respiratory function and acid-base balance.
Specimen: arterial blood (typically from the radial artery) collected in a heparinized syringe. The sample must be analyzed within 30 minutes or placed on ice (to slow cellular metabolism). Air bubbles must be expelled immediately because ambient air rapidly changes PO₂ and PCO₂.
Parameters and reference ranges:
- pH: 7.35–7.45
- PaCO₂: 35–45 mmHg
- PaO₂: 80–100 mmHg
- HCO₃⁻: 22–26 mEq/L
- Base excess: −2 to +2 mEq/L
- SaO₂: 95–100%
Interpreting acid-base disorders (the Henderson–Hasselbalch approach): The pH is determined by the ratio of HCO₃⁻ to PaCO₂.
- Respiratory acidosis: low pH, high PaCO₂ (hypoventilation).
- Respiratory alkalosis: high pH, low PaCO₂ (hyperventilation).
- Metabolic acidosis: low pH, low HCO₃⁻ (diabetic ketoacidosis, renal failure, diarrhea).
- Metabolic alkalosis: high pH, high HCO₃⁻ (vomiting, diuretics).
- Compensation: the opposing system (respiratory or renal) adjusts to bring pH toward normal. Look at the expected compensation — if the measured PaCO₂ or HCO₃⁻ differs from the expected compensation, a mixed disorder is present.
Venous blood gas (VBG): pH and HCO₃⁻ correlate well with ABG, but PaO₂ and PaCO₂ do not. VBG is used when arterial puncture is difficult and when only acid-base status is needed.
Lactate: measured on the same sample. Elevated lactate (>2 mmol/L) indicates tissue hypoxia and is a marker of sepsis severity.
Serum Protein Electrophoresis
Serum protein electrophoresis (SPEP) separates serum proteins into fractions based on their electrophoretic mobility in an alkaline buffer (pH 8.6). Proteins are visualized by staining (typically Amaranth or Coomassie Blue), and the relative percentage of each fraction is quantified by densitometry.
The five fractions (from anode to cathode):
- Albumin (55–65%): the largest peak. Decreased in liver disease, nephrotic syndrome, malnutrition.
- Alpha-1 globulin (2–4%): mainly alpha-1-antitrypsin. Decreased in alpha-1-antitrypsin deficiency.
- Alpha-2 globulin (7–12%): haptoglobin, alpha-2-macroglobulin, ceruloplasmin. Increased in inflammation.
- Beta globulin (8–15%): transferrin, complement C3, beta-lipoprotein. A narrow band may indicate a monoclonal protein.
- Gamma globulin (10–20%): immunoglobulins (IgG, IgA, IgM). Polyclonal increase in chronic inflammation; monoclonal band (M-protein) in multiple myeloma, Waldenström macroglobulinemia, MGUS.
M-protein detection: a sharp, narrow peak in the gamma or beta region indicates a monoclonal immunoglobulin produced by a plasma cell clone. The M-protein is confirmed and typed by immunofixation electrophoresis (IFE).
Immunofixation electrophoresis (IFE): after SPEP, the serum is electrophoresed again, and specific antibodies against IgG, IgA, IgM, kappa, and lambda are applied. IFE identifies the heavy chain class and light chain type of the M-protein. IFE is more sensitive than SPEP and is used to confirm and characterize monoclonal gammopathies.
Urine protein electrophoresis: for detecting Bence Jones proteins (free light chains) in multiple myeloma. A 24-hour urine collection is concentrated and electrophoresed. Free light chains appear in the gamma or beta region.
