pH is the negative logarithm of the hydrogen ion concentration and is an index of acidity and alkalinity of the blood. It changes nonlinearly masking magnitude of acid " “base disorders. The hydrogen ion concentration is dictated by the ratio of two quantities: the HCO3 ¢ ˆ ’ concentration, which is regulated by the kidneys, and the PCO2, which is controlled by the lungs.
Normal range:
Arterial: 7.35 " “7.45
Venous: 7.31 " “7.41
Use
To evaluate acid " “base disorders
Interpretation
Increased In
Metabolic alkalosis (plasma bicarbonate excess)
Excessive alkali administration
Potassium depletion (GI loss, lack of potassium intake, diuresis)
Stimulation of respiratory center by increased intracranial pressure
Hypoxia with normal overall alveolar diffusion of CO2
Fever
Salicylate poisoning (early)
Excessive artificial ventilation
Decreased In
Metabolic acidosis (bicarbonate deficit)
Increased formation of acids
Ketosis (DM, starvation, hyperthyroidism, high-fat low-carbohydrate diet, after trauma)
Cellular hypoxia including lactic acidosis
Decreased excretion of H+
Renal failure (prerenal, renal, and postrenal)
Renal tubular acidosis
Fanconi syndrome
Acquired (drugs, hypercalcemia)
Inherited (cystinosis, Wilson disease)
Addison disease
Respiratory acidosis
Emphysema, pneumonia, and pulmonary edema
Bronchoconstriction, plugs, and drugs depressing the respiratory center
Obstructive or restrictive pulmonary disease
Limitations
The pH of freshly drawn blood decreases on standing at a rate of 0.04 " “0.08 pH U/hour at 37 ‚ °C, by approximately 0.03 U/hour at 25 ‚ °C, but only 0.008 U/hour at 4 ‚ °C.