Genetic Testing and the IL-18 'Blood Clue'

Confirming a diagnosis of AIFEC is a two-part process: doctors look for biological “footprints” in the blood and then attempt to find the specific “glitch” in the NLRC4 gene. Because this condition is so rare and the genetics can be complex, a standard genetic test may sometimes come back negative even if a child truly has the disease.

Biological Footprints: The Role of IL-18

Before genetic results are in, a blood marker called IL-18 often provides supportive evidence. In children with AIFEC, the body’s “stuck-on” alarm system produces massive amounts of this inflammatory protein ^[1].

• Total vs. Free IL-18: Most labs measure “total” IL-18. However, the body also produces a “binding protein” that acts like a sponge to soak up the excess. It is the “free” (unbound) IL-18 that actually drives inflammation ^[1][2]. Testing for free IL-18 is highly specialized and not routinely available everywhere.
• A Diagnostic Clue, Not a Monitoring Tool: While IL-18 levels in AIFEC are often incredibly high (sometimes in the tens of thousands), they do not always go up and down with daily symptoms. A high IL-18 level provides context for an expert, but it cannot be used alone to track if a flare is starting or if a treatment is working ^[2][3].
• The “Look-Alike” Problem: Very high IL-18 is also seen in other rare conditions like XIAP deficiency or Systemic Juvenile Idiopathic Arthritis (sJIA). To be sure it is AIFEC, expert phenotype assessment and a genetic test are required ^[4][5].

Genetic Testing: Finding the “Glitch”

Doctors use Next-Generation Sequencing (NGS) to read the NLRC4 gene. This can be done through a targeted panel (which looks at a specific group of immune genes) or Whole Exome Sequencing (WES) (which reads all protein-coding genes) ^[6][7].

Why Tests Can Be Tricky: Somatic Mosaicism

Most genetic conditions are germline, meaning the mutation is in every single cell of the body. However, some children with AIFEC have somatic mosaicism ^[8]. In these cases, the mutation happened after the egg was fertilized, so it is only present in some cells (like some white blood cells) and not others ^[2][9].

If the percentage of mutated cells is very low, a standard genetic test might miss it ^[9][10]. If your child’s symptoms are classic but the test is negative, doctors may order deep sequencing or digital droplet PCR (ddPCR), which are much more sensitive tools designed to find these mutations ^[10][9]. However, a negative blood test does not definitively establish AIFEC, and testing another tissue is not a routine next step for every child—it is a specialized decision.

Understanding Your Genetic Report

When you receive your child’s report, you will encounter several technical terms that describe how the mutation was found:

• Heterozygous: This means the child has one normal copy of the NLRC4 gene and one mutated copy ^[6][11]. In AIFEC, one mutated copy is enough to cause disease (this is called autosomal dominant), but this only proves AIFEC if it’s an activating variant with a matching phenotype.
• De Novo: This means the mutation is new in the child and was not inherited from either parent ^[6][12].
• Pathogenic or Likely Pathogenic: These labels mean the lab is confident the mutation causes disease.
• VUS (Variant of Uncertain Significance): This means a change was found, but the lab isn’t sure yet if it’s harmful. A VUS alone does not confirm the diagnosis and should not be used to direct treatment. Specialized “functional studies” or parental testing may be needed to see if the mutation actually causes the NLRC4 protein to be overactive ^[13][9].

Genetic Completeness Checklist

Ensure your child’s genetic report or medical record includes the following details:

1. Gene Name: Confirmed as NLRC4.
2. Specific Variant: Listed as a “p.” code (e.g., p.Val341Leu) ^[2].
3. Variant Allele Fraction (VAF): If mosaicism is suspected, VAF is the proportion of sequencing reads containing the variant (Note: VAF is not always a direct cell percentage; ask your genetic counselor to interpret test sensitivity) ^[9].
4. Zygosity: Usually listed as “heterozygous” ^[11].
5. Inheritance: Whether it is “de novo” or inherited from a parent ^[6].