Approach to hypocalcemia: Clinical sciences

Hypocalcemia refers to a serum calcium level that is below the lower limit of normal, which varies among different labs but is often considered below 8.5 mg/dL. Calcium plays a vital role in various body functions, such as cardiac muscle function and nerve signaling. So, calcium imbalances such as hypocalcemia often result in abnormal cardiac rhythm and neurologic dysfunction. Some important causes of hypocalcemia that you should keep in mind include hypomagnesemia, impaired vitamin D conversion, hypoparathyroidism, and secondary hyperparathyroidism.

Now, if your patient presents with a chief concern suggesting hypocalcemia, you should first perform an ABCDE assessment to determine if your patient is unstable or stable. If the patient is unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access and put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry, as well as cardiac telemetry. Finally, if needed, provide supplemental oxygen.

Now, here’s a high-yield fact to keep in mind! If a patient has hypocalcemia, they may experience laryngospasm, seizures, or a prolonged QT interval on an ECG. This is especially true when the hypocalcemia is severe or develops rapidly. In such cases, administer intravenous calcium gluconate to increase blood calcium levels. Also, don’t forget that hypomagnesemia often occurs along with hypocalcemia, so consider giving IV magnesium in this situation as well.

Now that we're done with unstable patients, let’s go back to the ABCDE assessment and discuss the stable ones.

If your patient is stable, first obtain a focused history and physical examination, and order labs, including serum calcium and magnesium levels.

Your patient may report numbness around the mouth, paresthesias of the fingers and toes, and even emotional lability. On the other hand, physical exam can reveal positive Chvostek or Trousseau signs. The Chvostek sign is positive if tapping over the muscles overlying the facial nerve causes facial muscle spasms. On the other hand, the Trousseau sign is when inflating a blood pressure cuff over the patient’s arm causes a spasm of their hand. You can easily remember them as Chvostek for Cheek, and Trousseau for Triceps!

Then, the first thing you need to check with labs is calcium. Clearly, if results reveal a serum calcium level that’s below the lower limit of normal, you should consider, but not diagnose yet, hypocalcemia. This is because approximately half of the calcium in the blood is bound to plasma proteins, mainly albumin, while the other half circulates as ionized calcium. Although ionized calcium reflects the body's calcium stores accurately, measuring it is more complex than measuring total serum calcium. Also, keep in mind that when measuring total serum calcium in the setting of low serum albumin, there's a chance of getting false results, since the albumin-bound calcium is also affected.

So, to avoid false results and truly diagnose hypocalcemia, you might need to calculate the corrected total serum calcium level. Do this by subtracting the patient's serum albumin level from 4 and multiplying the difference by 0.8. Next, add the product to the measured serum calcium, and you will obtain the corrected calcium level. If the corrected serum calcium level is below the reference range, you can confirm the diagnosis of hypocalcemia!

Now, once you’ve diagnosed it, your next step is to review the serum magnesium level, and if it’s below the lower limit of normal, diagnose hypocalcemia due to magnesium deficiency. The thing is, magnesium is essential for the production of parathyroid hormone or PTH. In the setting of low serum magnesium, PTH synthesis is reduced, which in turn leads to hypocalcemia. However, by correcting magnesium levels, we can restore the normal production of PTH, eventually bringing serum calcium levels back to normal.

However, if the serum magnesium level is normal, you should order additional labs, including a basic metabolic panel; serum PTH; and 25-hydroxy vitamin D, which is actually the inactive form of vitamin D.