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MLT Registration: Clinical Chemistry

MLT Registration · Work through your exam, one topic at a time.

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Clinical Chemistry

Clinical chemistry questions combine analytical method knowledge with clinical interpretation. The MLT exam expects you to know how a result is produced (spectrophotometry, ion-selective electrodes, immunoassay), when the number is an artifact, and how to read it against a reference interval or an acid-base calculation.

Analytical principles

  • Beer-Lambert law: absorbance is proportional to concentration and path length, so results are only valid inside the analytically measurable range.
  • Indirect (diluted) ion-selective electrodes give falsely low sodium in severe hyperlipidemia or hyperproteinemia (pseudohyponatremia); direct ISE on whole blood does not.
  • Hemolysis raises potassium, LDH, AST and phosphate and interferes with several colorimetric assays; lipemia scatters light and bumps indirect ISE sodium down and triglycerides up.
  • Calibration establishes the relationship between signal and concentration; controls verify performance. Controls never substitute for calibration.

Quality control rules

  • 1-2s is a warning rule only; 1-3s, 2-2s, R-4s, 4-1s and 10x are rejection rules.
  • 2-2s, 4-1s and 10x indicate systematic error (shift, calibration drift, reagent lot change); R-4s indicates random error (pipetting, bubbles, electricity).
  • Six or more consecutive points on one side of the mean is a shift; six steadily rising or falling points is a trend.

Interpretation anchors

  • Anion gap = Na - (Cl + HCO3), roughly 8-16 mmol/L.
  • Corrected sodium = measured Na + 1.6 mEq/L for every 100 mg/dL glucose above 100.
  • Friedewald LDL = total cholesterol - HDL - (triglycerides/5); invalid when triglycerides exceed about 400 mg/dL, so use a direct LDL.
  • AST:ALT above 2 favors alcohol-related liver injury; an isolated ALP rise with high GGT supports a biliary source.
  • Delayed separation or a non-fluoride tube lowers glucose by glycolysis and raises potassium.

Worked example

Sodium 132, chloride 98, bicarbonate 10, glucose 600 mg/dL, arterial pH 7.25. Corrected sodium = 132 + 1.6 x 5 = 140. Anion gap using measured sodium = 132 - 108 = 24; using corrected sodium = 140 - 108 = 32. Delta ratio = (24 - 12)/(24 - 10) = 0.86, near 1, indicating a pure high-anion-gap metabolic acidosis. With glucose 600 mg/dL and ketones present, the picture is diabetic ketoacidosis. Reasoning: hyperglycemia dilutes measured sodium, so the gap must be interpreted with the correction in mind.

Traps that cost marks

  • Reporting a normal anion gap in a hyperglycemic patient without correcting sodium.
  • Calculating LDL when triglycerides are 500 mg/dL.
  • Treating a 1-2s control value as a rejection.
  • Reading a hemolyzed potassium as true hyperkalemia.