From the Guidelines
The best test to check magnesium levels is not a serum magnesium blood test due to its inaccuracy, as it only measures less than 1% of the body's magnesium stores. According to the British Society of Gastroenterology consensus guidelines on the management of inflammatory bowel disease in adults 1, serum magnesium is not a reliable indicator of magnesium status. This is because the majority of magnesium is stored in bone, soft tissue, and muscle, making serum magnesium levels a poor reflection of the body's overall magnesium stores.
When assessing magnesium levels, it's essential to consider the clinical presentation and symptoms of magnesium deficiency, such as abdominal cramps, impaired healing, fatigue, and bone pain, as reported in the guidelines 1. In some cases, a magnesium loading test or RBC magnesium test may be recommended for a more comprehensive assessment. However, these tests are not commonly used as the first-line approach.
It's crucial to note that magnesium deficiency is common in patients with inflammatory bowel disease (IBD), affecting 13-88% of patients, and is often caused by increased gastrointestinal losses 1. Therefore, a thorough clinical evaluation and consideration of the patient's medical history and symptoms are necessary to determine the best course of action for assessing and managing magnesium levels.
In clinical practice, healthcare providers may need to rely on a combination of clinical judgment, medical history, and laboratory tests to diagnose and manage magnesium deficiency, rather than relying solely on a serum magnesium blood test.
From the FDA Drug Label
The normal serum level is 1.5 to 2.5 mEq/L. Magnesium sulfate injection should not be given unless hypomagnesemia has been confirmed and the serum concentration of magnesium is monitored.
The best test to check magnesium levels in the blood is serum magnesium level testing, which measures the concentration of magnesium in the blood.
- This test is essential to confirm hypomagnesemia and to monitor the serum concentration of magnesium during treatment with magnesium sulfate injection.
- The normal serum level of magnesium is between 1.5 to 2.5 mEq/L 2.
From the Research
Magnesium Level Tests
To determine the best test for checking magnesium levels in the blood, several options are considered:
- Serum magnesium concentration (SMC) is the most commonly used test, but it has limitations due to the large number of "normal" individuals with subtle chronic negative magnesium balance 3.
- Total magnesium in tissues and physiologic tests may provide important information, but there is no simple, rapid, and accurate test to indicate total body magnesium status 4, 5.
- Ionized magnesium (iMg) is considered a more useful indicator of magnesium status, especially in critically ill patients, as it reflects the biologically active fraction of magnesium 6, 7.
Comparison of Magnesium Tests
The following points compare the different tests:
- Serum total magnesium may overestimate the incidence of hypomagnesemia, especially in patients with hypoalbuminemia, whereas ionized magnesium measurement may be more accurate 7.
- The correlation between serum ionized and total magnesium is significant, but the fraction of ionized magnesium can vary depending on the serum albumin level 7.
- Measuring ionized magnesium instead of total magnesium may be advantageous in evaluating patients with hypoalbuminemia or expected hypomagnesemia 7.
Recommendations
Based on the available evidence: