Reference values — Ionized calcium (serum)
Ionized calcium is the physiologically active fraction of serum calcium and is typically unchanged in normal pregnancy despite hemodilutional decreases in total calcium. Interpretation should account for pH, albumin levels, and maternal symptoms.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mg/dL | 4.5 – 5.3 | 4.5 – 5.1 | 4.4 – 5.0 | 4.4 – 5.3 |
| mmol/L | 1.13 – 1.33 | 1.13 – 1.28 | 1.10 – 1.25 | 1.10 – 1.33 |
Pregnancy-specific considerations
Total serum calcium decreases due to hemodilution and lower albumin, but ionized calcium remains stable. Measurement is most useful when evaluating parathyroid disorders, vitamin D deficiency, electrolyte abnormalities, or neuromuscular symptoms.
Causes of elevated ionized calcium
- Primary hyperparathyroidism
- Malignancy (PTHrP-mediated or osteolytic)
- Granulomatous disease (sarcoidosis, TB)
- Vitamin D intoxication
- Thyrotoxicosis
- Adrenal insufficiency
- Thiazide diuretics
- Milk-alkali syndrome
- Immobilization
Causes of low ionized calcium
- Hypoparathyroidism
- Vitamin D deficiency
- Hypomagnesemia
- Sepsis or critical illness
- Acute pancreatitis
- Renal insufficiency
- Massive transfusion (citrate binding)
- Respiratory alkalosis
- Preeclampsia (mild transient association)
Low ionized calcium may cause paresthesias, tetany, laryngospasm, or arrhythmias and should be promptly evaluated.
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114(6):1326–1331.
- Wallach J. Interpretation of Diagnostic Tests. 8th ed. LWW; 2007.
- Fischbach FT, Dunning MB. Manual of Laboratory and Diagnostic Tests. 7th ed. LWW; 2004.