Proving the Diagnosis: Tests and Lab Results for RALD

Confirming a diagnosis of RALD is a complex process of “proving a presence” while “ruling out others.” Because RALD shares symptoms with several other conditions, your doctors must use a combination of genetic, cellular, and physical evidence to ensure the diagnosis is accurate.

The Molecular Clue

An important molecular clue for RALD is finding a mutation in the KRAS or NRAS genes in your blood cells ^[1]. However, simply finding the mutation isn’t enough; activating RAS-pathway variants can also occur in JMML, other myeloid neoplasms, and clonal hematopoiesis. The mutation alone does not confirm RALD; doctors must also determine where it is and where it isn’t, and evaluate the full clinical phenotype.

• Somatic vs. Germline Testing: This is the most critical distinction. Your doctor will test your blood (where the mutation should be) and compare it to a “non-blood” tissue ^[1][2].
• Why it matters: If the mutation is only in the blood, it is somatic (acquired), which points to RALD ^[1]. If it is in every tissue, it is germline (inherited), which suggests a different category of diseases called RASopathies ^[1]. However, buccal swabs and saliva can contain substantial amounts of blood leukocytes (white blood cells), meaning they can be contaminated by the blood clone and give false reassurance. A validated specimen strategy, often guided by a genetics specialist using cultured skin fibroblasts, may be needed in select cases to rule out constitutional mosaicism.
• Variant Allele Frequency (VAF): This number tells doctors the percentage of sequenced alleles carrying the variant, which is only an indirect estimate of how many blood cells carry the mutation. In limited reported cohorts of RALD, this often ranges between 8% and 43% ^[3].

Laboratory Findings: The Blood and Bone Marrow

Your medical team will look for a specific pattern in your lab reports that distinguishes RALD from its “look-alikes.”

The CBC and Peripheral Smear

A Complete Blood Count (CBC) with a differential is the first step. In RALD, this typically shows persistent monocytosis—an absolute monocyte count that stays high (usually ≥ 1.0 x 10⁹/L) ^[4][5]. Note that while a threshold of at least three months is often discussed, this is closely associated with JMML diagnostic frameworks rather than a standalone RALD criterion. Doctors also look at a “smear” (blood under a microscope) to ensure there aren’t too many immature “blast” cells, which would point toward leukemia ^[5].

Flow Cytometry

This test acts like a “roll call” for your immune cells. In RALD, flow cytometry often finds an increase in double-negative T cells (DNTs) ^[6]. While DNTs are also the hallmark of ALPS, RALD patients often have a unique “signature” that includes activated monocytes and a specific type of B-cell (polyclonal CD10+ B cells) ^[4].

The Bone Marrow Biopsy

While a blood test can often find the mutation, a bone marrow biopsy is frequently performed to rule out JMML (Juvenile Myelomonocytic Leukemia).

• What they look for: In RALD, the bone marrow usually looks relatively “normal” or shows a slight increase in monocytes, whereas in JMML, the marrow is often very “crowded”. Keep in mind, however, that JMML can sometimes have normal cytogenetics, and RALD can show marrow abnormalities; no single result rules either condition in or out ^[2][5].
• Deep Testing: In rare cases, the mutation might not show up in the blood but can be found by testing the bone marrow directly ^[7].

Differentiating from ALPS

To ensure the condition is not ALPS (Autoimmune Lymphoproliferative Syndrome), doctors check for several “biomarkers” that are often high in ALPS ^[8]:

• Vitamin B12: Extremely high levels are often seen in ALPS ^[9].
• Soluble FAS Ligand (sFasL): High levels suggest a defect in the FAS pathway ^[8].

These tests can overlap between disorders and cannot replace expert clinical, functional, and genetic evaluation. Double-negative T cells can also occur outside of classic ALPS.

Tests That May Be Considered for Diagnosis

A complete diagnostic workup for RALD may include the following data points.