Diagnostic Tests & Avoiding Misdiagnoses

Diagnosing a rare condition like Chronic Visceral ASMD can be a complex process, often requiring a “detective work” approach by specialists. Because its symptoms—like an enlarged spleen and low platelets—overlap with more common conditions, it is frequently misdiagnosed ^[1]. Getting to the right answer requires a combination of enzyme testing, genetic analysis, and specialized biomarkers interpreted by an expert.

The Diagnostic Pathway

Confirming ASMD generally involves a two-step approach ^[2]:

1. Enzyme Activity Testing: Measuring the activity of the acid sphingomyelinase (ASM) enzyme using a validated assay on a dried blood spot, white blood cells (leukocytes), or skin cells (fibroblasts) ^[3]. In people with ASMD, this activity is significantly reduced. It must be interpreted alongside the laboratory’s specific reference range ^[2].
2. Genetic Testing: Sequencing the SMPD1 gene to identify pathogenic variants (mutations) ^[4]. To have ASMD, a person must have two disease-causing variants—one inherited from each parent. A geneticist must also determine if these variants are “in trans” (on opposite copies of the gene) ^[5].

A Critical Warning: The “False-Normal” Result

One of the biggest challenges in diagnosing ASMD is that some common laboratory tests can occasionally miss the disease. Some labs use synthetic substrates (artificial chemicals) to measure enzyme activity ^[6].

(Note for your clinician: In certain genetic types of ASMD—specifically those involving variants like Q294K, C92W, or P184L—the enzyme may appear to work “normally” when tested with these artificial substances, leading to a false-normal result ^[7][6].)

If your symptoms strongly suggest ASMD but your enzyme test comes back normal, it is vital to follow up with genetic testing or biomarker measurement ^[7][8]. Furthermore, genetic testing sometimes reveals “variants of uncertain significance” (VUS), where it is not immediately clear if the variant causes disease. A genetic counselor is essential for interpreting these nuances and organizing testing for siblings or other relatives.

The Role of Plasma Biomarkers

Recently, doctors have begun using specialized blood markers to help confirm a diagnosis and monitor disease burden, especially when enzyme results are borderline ^[3].

• Lyso-SM (lysosphingomyelin): This is a breakdown product of the fat that builds up in ASMD. It is highly elevated in patients with ASMD and is an established biomarker for the disease ^[9][10].
• Lyso-SM-509: Another marker that can be elevated. However, examining the Lyso-SM-509 to Lyso-SM ratio is chiefly used to help distinguish Niemann-Pick Type C (NPC) from ASMD ^[11].

Biomarkers are supportive tools and do not replace formal enzyme and genetic testing.

Avoiding Common Misdiagnoses

ASMD is often mistaken for other “storage” diseases because they share similar features like an enlarged spleen (splenomegaly) ^[12].

Gaucher Disease

Gaucher disease is the most common misdiagnosis for ASMD ^[13]. Both cause an enlarged spleen and low platelets. To tell them apart, doctors should perform “parallel testing,” measuring both ASM and the enzyme for Gaucher (glucocerebrosidase) at the same time ^[13]. Key clues that strongly support ASMD rather than Gaucher include the presence of interstitial lung disease and very low HDL cholesterol ^[12][14].

Niemann-Pick Type C (NPC)

Despite the similar names, ASMD and NPC are caused by different biological problems. While ASMD is an enzyme deficiency, NPC is a problem with how cholesterol moves within cells ^[11]. Doctors can distinguish them using the Lyso-SM-509/Lyso-SM ratio and molecular testing. Additionally, NPC often involves progressive neurological symptoms, like difficulty looking up and down, which are not typical in Chronic Visceral ASMD ^[15].