40 Questions
What may occur as a result of rapid remineralization of bone after surgery or primary hyperparathyroidism?
Hypocalcemia
Which of the following is a cause of hypercalcemia?
Increased skeletal resorption
What is the best indicator of calcium status?
Free calcium
Which of the following methods is used to measure total serum calcium concentrations?
All of the above
What is the preferred specimen for the measurement of total calcium?
Serum and heparinized plasma
What is commonly used for the rapid measurement of free calcium?
Ion-specific electrode (ISEs)
What affects the binding of calcium by protein and small anions in vitro and in vivo?
pH
What is a consequence of vitamin D deficiency?
Hypocalcemia
What percentage of the body's calcium is stored in the skeleton?
99%
What percentage of calcium in plasma is bound to plasma proteins?
40%
What is the primary mechanism that regulates calcium concentration in plasma?
PTH and 1,25(OH)2D
What is a key role of intracellular calcium?
All of the above
What is a major function of extracellular calcium?
Bone mineralization
What is a cause of hypocalcemia?
Reduction in the albumin-bound calcium
What is a common clinical condition associated with low serum albumin?
Chronic liver disease
What is the term for the condition where total calcium is decreased, but free calcium is normal?
Pseudohypocalcemia
What is the effect of increasing the pH of a specimen in vitro on protein-bound calcium?
It increases protein-bound calcium
What is the effect of decreasing the pH of a specimen in vitro on free calcium?
It increases free calcium
How much does free calcium change for each 0.1 unit change in pH?
It changes by 5%
Why should specimens be analyzed at the patient's in vivo pH?
Because of the inverse relationship between free blood calcium and pH
What is the purpose of collecting and handling specimens anaerobically?
To minimize alterations in pH and free calcium
Why should syringes and evacuated tubes be filled completely and sealed?
To prevent the loss of CO2 and increase in pH
How long is free calcium stable in whole blood specimens at room temperature?
For 1 hour
What should be reported on each specimen?
Both the free calcium concentration and the actual pH of the specimen
What is a common source of preanalytical error in calcium measurement?
Tourniquet use and venous occlusion during sampling
What is the effect of forearm exercise on pH and free calcium?
Decreases pH and increases free calcium
What is the reference interval for total calcium in adults?
2.15 to 2.57 mmol/L
What is the primary location of organic phosphate in blood?
Primarily within cells
What is the effect of hyperventilation on free calcium?
Decreases free calcium
What is the form of phosphate measured in serum and plasma by clinical laboratories?
Inorganic phosphate
What is a major component of hydroxyapatite in bone?
Phosphate
What is the effect of tourniquet use on total calcium?
Increases total calcium
Why are hemolyzed specimens unacceptable?
They contain high concentrations of organic phosphate esters.
What is the effect of hemolysis on inorganic phosphate concentration in specimens?
It increases inorganic phosphate concentration.
What is the approximate percentage of total body magnesium found in the skeleton?
55%
What is the role of magnesium in enzyme systems?
It is an allosteric activator of many enzyme systems.
What is the effect of reducing serum magnesium concentration on neuromuscular excitability?
It increases neuromuscular excitability.
What is a common cause of magnesium deficiency?
All of the above.
What is a common association with magnesium deficiency?
All of the above.
What is the normal reference interval for serum phosphate in adults?
0.81 to 1.45 mmol/L.
Study Notes
Calcium
- Calcium is the fifth most common element in the body and the most prevalent cation.
- The skeleton contains 99% of the body's calcium.
- Calcium exists in three physicochemical states in plasma:
- 50% is free (ionized)
- 40% is bound to plasma proteins, primarily albumin
- 10% is complexed with small anions
- Calcium is redistributed among these three plasma pools, acutely or chronically, by:
- Alterations in the concentration of protein and small anions
- Changes in pH
- Changes in the quantities of free calcium and total calcium in the serum
Regulation of Calcium
- Calcium concentration in plasma is tightly regulated by hormones:
- Parathyroid hormone (PTH)
- 1,25(OH)2D
- Intracellular calcium has key roles in many important physiological functions, including:
- Muscle contraction
- Hormone secretion
- Glycogen metabolism
- Cell division
- Extracellular calcium is needed for:
- Bone mineralization
- Blood coagulation
- Stabilizing plasma membranes and influencing permeability and excitability
Hypocalcemia
- Hypocalcemia (low total serum calcium) may be due to:
- A reduction in the albumin-bound calcium
- A reduction in the free fraction of calcium, or both
- Common clinical conditions associated with low serum albumin include:
- Chronic liver disease
- Nephrotic syndrome
- Congestive heart failure
- Malnutrition
Measurement of Calcium
- Methods used to measure total serum concentrations of calcium include:
- Spectrophotometric methods
- Ion-specific electrode (ISE) methods
- Atomic absorption methods
- Samples should be collected and handled anaerobically to minimize alterations in pH and free calcium.
- Free calcium is measured in heparinized whole blood, and is reported in mmol/L.
Phosphate
- Phosphorus in the form of inorganic and organic phosphate is an important and widely distributed element in the human body.
- Inorganic phosphate is the fraction measured in serum and plasma by clinical laboratories.
- Phosphate in plasma exists as anions in the form of:
- Monovalent (H2PO4)
- Divalent (HPO4)
- Phosphate is a major component of hydroxyapatite in bone and is also found in soft tissue.
- Most of the phosphate in soft tissue is incorporated into nucleic acids, phospholipids, phosphoproteins, and high-energy compounds, such as adenosine triphosphate (ATP).
Interferences and Reference Intervals
- Hemolysis, icteric, and lipemic specimens can interfere with phosphate measurement.
- Reference interval for serum phosphate in adults is 0.81 to 1.45 mmol/L.
Magnesium
- Magnesium is the fourth most abundant cation in the body, with approximately 55% in the skeleton and 45% intracellular.
- Magnesium functions as a cofactor for more than 300 enzymes and is required for enzyme-substrate complex formation.
- Magnesium is an allosteric activator of many enzyme systems and competitively inhibits the entry of calcium into neurons.
- Reducing the serum magnesium concentration results in increased neuromuscular excitability.
Hypomagnesemia/Magnesium Deficiency
- Hypomagnesemia often reflects a shift of magnesium into cells without replacement.
- Moderate or severe magnesium deficiency is usually due to gastrointestinal (GI) tract or kidney losses.
- Magnesium deficiency is commonly associated with:
- Diarrhea
- Excessive urinary losses of magnesium from the kidneys in:
- Alcoholism
- Diabetes mellitus (osmotic diuresis)
This quiz covers the regulation of calcium levels in the body, including hypocalcaemia and hypercalcemia, and their causes and effects.
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