Lab Tests & Genetic Testing: Understanding Your Child's Report

Navigating the laboratory and genetic reports for Familial hypomagnesemia with secondary hypocalcemia (HSH) can feel like learning a new language. These reports provide the evidence needed to distinguish HSH from more common conditions like epilepsy or primary parathyroid disorders. To confirm a diagnosis of HSH, doctors look for a specific “fingerprint” in the blood, urine, and DNA ^[1][2].

The Biochemical Profile of HSH

When a child has untreated HSH, their blood work typically shows a very specific pattern. Understanding these markers helps explain why the body is struggling. However, clinicians must interpret these together, as no single test is perfect:

• Magnesium (Mg): In HSH, magnesium is “profoundly” low ^[2]. This is the primary driver of all other symptoms.
• Calcium (Ca): This is also low, which is why HSH is often misdiagnosed as a calcium disorder ^[1][3].
• Parathyroid Hormone (PTH): This hormone is usually low or “inappropriately normal” ^[1]. Normally, when calcium is low, PTH should be very high to fix it. In HSH, the low magnesium prevents the body from making or responding to PTH ^[4].
• Phosphorus (Phos): This is typically normal or slightly high, which helps differentiate HSH from other bone-related disorders ^[1].
• Vitamin D: Levels of 25-hydroxyvitamin D are usually normal or even high, ruling out nutritional rickets as the primary cause ^[1].

The FEMg Test: Checking the Kidneys

One of the most important tools in an HSH diagnosis is the Fractional Excretion of Magnesium (FEMg). This is not a single blood test but a calculation that compares the amount of magnesium in the blood to the amount being lost in the urine ^[5].

To get an accurate result, doctors must collect a blood sample and a urine sample at the same time. In a child with severe magnesium deficiency from HSH, the kidneys are working perfectly—they see the body is starving for magnesium and try to save every bit of it. Therefore, the FEMg in an HSH patient is typically 2% or lower ^[2][1].

• If FEMg is <= 2%: This indicates the magnesium is being conserved by the kidneys, supporting an extrarenal loss like the gut ^[2].
• If FEMg is > 4%: This suggests the kidneys are “wasting” magnesium, which points toward a different genetic kidney condition rather than HSH ^[6].

Clinical Caveat: A low FEMg mainly shows that the kidneys are conserving magnesium and does not by itself establish HSH. Interpretation depends on age, kidney function, and other clinical context, requiring cautious clinician interpretation.

Genetic Testing: The Definitive Answer

While blood and urine tests point to HSH, genetic testing of the TRPM6 gene provides the final confirmation. Because HSH is an autosomal recessive condition, the report must ideally show two pathogenic or likely pathogenic mutations (one from each parent) ^[7][8].

Types of Genetic Analysis

Standard genetic “sequencing” (reading the DNA code like a book) is the first step, but it doesn’t always catch everything. A complete report should consider:

1. Sequencing: Looks for “spelling errors” in the 39 sections (exons) of the TRPM6 gene ^[9].
2. Deletion/Duplication Analysis (MLPA): This looks for missing “pages” or “chapters” of the gene. Some children have HSH because a large chunk of the gene is entirely absent, which standard sequencing can miss ^[4][10].
3. Splice-Site Analysis: This checks the “punctuation” between the gene’s sections. If this is broken, the body cannot assemble the magnesium channel correctly ^[4][10].

Parental Segregation Testing

Once a mutation is found in the child, doctors often perform parental segregation testing ^[11]. This simply means testing both parents to ensure each one carries one of the child’s mutations. This confirms that the child inherited the condition “in trans” (one from mom and one from dad), which is the hallmark of a recessive genetic diagnosis ^[4][8]. A negative panel does not necessarily exclude HSH if the phenotype is strong, and a Variant of Uncertain Significance (VUS) requires further interpretation by a genetics professional.

Lab Report Completeness Checklist

Use this list to ensure your child has had a comprehensive diagnostic workup:

• [ ] Paired Blood and Urine: Were serum and urine magnesium/creatinine collected simultaneously?
• [ ] FEMg Calculation: Is the FEMg recorded? (Target for HSH is typically ≤2%) ^[2].
• [ ] Comprehensive Genetic Panel: Did the test include both sequencing and deletion/duplication analysis for the TRPM6 gene? ^[4].
• [ ] PTH Level: Was the Parathyroid Hormone level checked while calcium was low? ^[1].
• [ ] Kidney Function: Was creatinine or eGFR checked? (Kidney health affects how we interpret the FEMg) ^[12].
• [ ] Medication Review: Was the child taking any diuretics or other medications that could interfere with magnesium levels during testing? ^[13].