Clinical Laboratory - Chemical Pathology and Immunology

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Metabolic Testing

Electrolytes: the standard electrolytes are electrically conductive ions and include sodium (Na), potassium (K), chloride (Cl), and bicarbonate (HCO3). Calcium (Ca), phosphorus (as PO4), and magnesium (Mg) may be measured as indicated.

The most important cations are Na and K. The concentration of Na is greatest in extracellular fluid, while the concentration of K is highest within cells. Of the electrolytes listed, the concentrations for the anions Cl and HCO3 are greatest extracellularly. Within the cell proteins add considerably to the negative charge.

The concentrations of electrolytes help to determine the state of hydration, acid-base balance, and renal function.

Osmolality: the osmolality is the concentration of particles in a fluid, generally measured in plasma and in urine. The higher the number of particles (cations, anions, proteins) in solution, the higher the osmolality. The osmolality gives an indication of hydration and of renal function. A high osmolality is indicative of dehydration while low osmolality may represent fluid overload from lack of renal excretion of free water.

Blood gases: the standard blood gas analysis is performed on arterial blood (radial artery) and measures the very small amount of dissolved oxygen and carbon dioxide in arterial blood, given as partial pressures: PaO2 and PaCO2, given as mm Hg. In addition, the pH of the blood and the bicarbonate concentration (HCO3) are measured. Most blood gas analyzers will also provide a measurement for dissolved carboxyhemoglobin (CO). If the patient is receiving supplemental oxygen, then the concentration of inspired oxygen may be higher. Therefore:

  • The ability of the lungs to oxygenate blood is measured by the PaO2

  • The amount of oxygen carried in the blood is a function of the amount of hemoglobin.

  • The amount of oxygen deliverable to tissues is a function of the oxygen saturation.

Acid-base balance: the acid-base status can be determined from blood gas analysis in the context of electrolyte concentrations. In general, there are four main alterations of acid-base status, based upon the hydrogen ion (H+) concentration in the plasma:

  • Respiratory acidosis: the respiratory capacity, effort, or rate is decreased, so that CO2 is retained and more H+ is present.

  • Respiratory alkalosis: the respiratory rate is increased so that more CO2 is eliminated and there is less H+ in the plasma.

  • Metabolic acidosis: there is a decrease in HCO3 with gain in H+.

  • Metabolic alkalosis: there is an increase in HCO3 with loss of H+.

By comparison to electrolyte concentrations, it can be determined if there is an acidosis with or without an "anion gap". The simplest formula:

anion gap = Na - (HCO3 + Cl)

In general, this gap ranges from 8 to 16 mEq/L, with an average of 12. An increased anion gap suggests an increase in some anion (lactate with poor tissue perfusion, ketoacids with diabetic ketoacidosis, sulfates and phosphates from renal failure).

Don't perform repetitive chemistry testing in the face of clinical and lab stability (from the Society for Hospital Medicine - Adult Hospital Medicine) Before ordering a test, consider what you would do differently because of the test result.

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