Fall in ionised calcium
Stimulus · 1.15-1.30 mmol/L ~50% of total
Total plasma calcium is 2.20-2.60 mmol/L, but only the free ionised half is biologically active and only that fraction is sensed. A fall of a few hundredths of a millimole per litre in ionised calcium is enough to release stored PTH within seconds.
Detail
- Distribution
- ~50% ionised (1.15-1.30 mmol/L), ~40% bound to protein (mostly albumin), ~10% complexed with citrate, phosphate and bicarbonate
- Correction
- Corrected calcium (mmol/L) = measured calcium + 0.02 x (40 - albumin in g/L); use it whenever albumin is abnormal, though a measured ionised calcium settles the question
- pH
- Alkalosis increases calcium binding to albumin and lowers ionised calcium with no change in total calcium; acidosis does the reverse
- Body pool
- About 99% of the body's ~1 kg of calcium is in bone as hydroxyapatite; the extracellular pool is tiny and tightly defended
- Response times
- Stored PTH within seconds to minutes, increased PTH gene transcription over hours, chief-cell hyperplasia over weeks
When it goes wrong
Acute respiratory alkalosis from hyperventilation or a panic attack
Ionised calcium falls while total calcium stays normal: perioral and digital paraesthesia, carpopedal spasm and a positive Trousseau sign
Massive transfusion, or citrate anticoagulation during apheresis or continuous renal replacement therapy
Citrate chelates ionised calcium: tingling, tetany, prolonged QT and hypotension despite a normal total calcium
Hypoalbuminaemia in nephrotic syndrome, cirrhosis or critical illness
Total calcium reads low while ionised calcium and the patient are normal; correct for albumin before treating
Practise this structure
2 questions in the bank tagged Ionised Calcium.
A 22-year-old woman is brought to the emergency department 30 minutes after an argument at home, breathing rapidly and deeply. She complains of tingling around…
Foundations · middle twist
A 22-year-old woman is brought to the emergency department after 30 minutes of rapid, deep breathing that began during an argument. She reports tingling around…
Foundations · middle twist