From the Guidelines
To diagnose suspected hypocalcemia, the primary lab test is a serum total calcium level, ideally paired with an ionized calcium measurement, as recommended by the most recent guidelines 1. The diagnosis of hypocalcemia requires a comprehensive approach, including laboratory tests and clinical evaluation. Key laboratory tests for suspected hypocalcemia include:
- Serum total calcium level
- Ionized calcium measurement, which is the physiologically active form and provides the most accurate assessment of calcium status, especially in patients with abnormal protein levels
- Serum albumin, to calculate corrected calcium if ionized calcium is unavailable
- Phosphorus
- Magnesium
- Parathyroid hormone (PTH)
- Vitamin D (25-hydroxyvitamin D)
- Kidney function tests (BUN, creatinine) These tests help determine the cause of hypocalcemia, which could be due to vitamin D deficiency, hypoparathyroidism, kidney disease, or other conditions, as discussed in the guidelines for managing adults with 22q11.2 deletion syndrome 1. An electrocardiogram (ECG) may also be warranted to assess for cardiac manifestations of hypocalcemia, such as prolonged QT interval, as hypocalcemia can be associated with cardiac arrhythmias and other complications 1. When interpreting results, it is essential to remember that total calcium must be adjusted for albumin levels, as approximately 40% of calcium is bound to albumin, and for every 1 g/dL decrease in albumin below 4 g/dL, add 0.8 mg/dL to the measured total calcium value, as noted in the clinical practice guidelines for bone metabolism and disease in chronic kidney disease 1. Severe hypocalcemia (ionized calcium <0.8 mmol/L or corrected total calcium <7.5 mg/dL) requires urgent evaluation and treatment, especially if the patient is symptomatic with tetany, seizures, or cardiac arrhythmias, emphasizing the importance of prompt diagnosis and management to prevent morbidity and mortality.
From the FDA Drug Label
The usual adult dosage in hypocalcemic disorders ranges from 200 mg to 1 g (2 -10 mL) at intervals of 1 to 3 days depending on the response of the patient and/or results of serum ionized calcium determinations. The laboratory test for suspected hypocalcemia (low calcium levels) is serum ionized calcium determinations 2.
From the Research
Laboratory Test for Suspected Hypocalcemia
The laboratory test for suspected hypocalcemia involves measuring the serum calcium level.
- A normal serum calcium level is 8 to 10 mg/dL 3
- The diagnosis of hypocalcemia is confirmed when the serum calcium level is less than 8 mg/dL 4
- Measurement of ionized calcium is the preferred way to ascertain the diagnosis of hypocalcemia 5
- Other laboratory tests that may be useful in the diagnosis of hypocalcemia include:
- Parathyroid hormone (PTH) level to differentiate between PTH-mediated and non-PTH mediated causes of hypocalcemia 3, 6, 4
- Serum phosphate level to help identify the underlying cause of hypocalcemia 6
- Vitamin D level to assess for vitamin D deficiency, which is a common cause of hypocalcemia 3, 4
- Magnesium level to assess for concomitant magnesium deficiency, which can contribute to hypocalcemia 3