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Diagnostic procedures (Proceedings)

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Aug 01, 2011

Calcium

     • Bone mineralization (skeletal structure)
     • Ion transport across cells
     • Cell contractility and secretion
     • Hormone secretion
     • Cell growth and division
     • Neuromuscular transmission

Serum calcium

     • 55% bound to albumin
     • 35% ionized (active and regulated) form
     • 10% "complexed" (to citrate, lactate, bicarbonate, phosphate)

Serum biochemical profile measures total calcium

Parathyroid hormone

     • Parathyroid gland = main endocrine organ involved in control of Ca/P metabolism
     • PTH actions
          o Ca and P reabsorption from bone
          o Increases Ca but decreases P reabsorption in kidneys
               • Potent phosphaturic action*
          o Increases activity of vit. D
               • Increased absorption of Ca and P in GI
     • Principle hormone in fine minute-to-minute blood calcium regulation
          o Released in response to hypocalcemia
          o Release inhibited by increased iCa, vit D

Vitamin D

     • Activated vitamin D (calcitriol) increases Ca concentration by increasing absorption of Ca from the intestine, and by enhancing PTH action on bone and kidney.
     • Cholecalciferol Calcidiol Calcitriol
     • Vitamin D actions
          o Increases GI absorption of Ca and P **
          o Increases bone reabsorption
     • Increases renal reabsorption

Note: pH effects

          o ACIDOSIS increases IONIZED CALCIUM
          o ALKALOSIS decreases IONIZED CALCIUM

Analytes for calcium status

     • Total calcium
     • Ionized calcium
     • Inorganic phosphorus
     • Parathyroid hormone (PTH)
     • Parathyroid hormone related peptide (PTHrp)
     • Vitamin D

Disorders of calcium

     • Hypocalcemia
     • Hypercalcemia
       (refer to both total calcium abnormalities and ionized calcium abnormalities)

Hypocalcemia: hypoalbuminemia

     • 55% of calcium is bound to albumin
     • When albumin decreases, TOTAL calcium decreases
     • Ionized calcium stays the same
     • Benign
     • Very common: most common cause of decreases in total calcium!

Correction formulas

     • Use of correction formulas is not recommended
     • Ionized calcium should be measured if true hypocalcemia is suspected
     • (that said, if albumin is low, it is likely the reason for the hypocalcemia)

Common causes of true hypocalcemia

     • Renal failure (acute and chronic)
     • Pancreatitis
     • Eclampsia

Renal failure and hypocalcemia

     • Decreased calcitriol formation by kidney
          o Decreased intestinal absorption of calcium
          o PTH is less effective at releasing Ca from bone
     • Increased phosphorus will decrease Ca (mass action, less important)

Eclampsia

     • 1-3 weeks post partum
     • Small breed dogs
     • Loss of calcium in milk/skeletal development
     • Seizures, trembling, twitching, shaking, and stiffness
     • Treatment is IV calcium gluconate