Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1104 page)

   Low-normal or suppressed intact PTH (<20 pg/mL) excludes hyperparathyroidism.

   Serum 1,25-dihydroxyvitamin D is usually decreased or low-normal in HHM but is increased in HPT.

   Limitations

   Production of PTHrP by the fetoplacental unit can cause transient increase during pregnancy, especially in the third semester.

   Primary hyperparathyroidism occurs in ≤10% of patients with HHM as well as in those receiving thiazides or with other causes of hypercalcemia.

PARTIAL PRESSURE OF CARBON DIOXIDE (pCO
₂
),
BLOOD

   Definition

   pCO
₂
is a measure of tension or pressure of carbon dioxide dissolved in the blood. The pCO
₂
of blood represents the balance between cellular production of CO
₂
and ventilatory removal of CO
₂
. A normal, steady pCO
₂
indicates that the lungs are removing CO
₂
at about same rate as tissues producing CO
₂
. A change on pCO
₂
indicates an alteration in this balance, usually due to the change in ventilatory status.

   
Normal range:

   Arterial: 35–45 mm Hg

   Venous: 41–51 mm Hg

   Interpretation

Increased In

   Acute respiratory acidosis

   Depression of respiratory center