Table 4 Key features of diagnostic methods for ultra-rare IEMs: NP-C as an example

Biomarkers

• Oxysterols (C-triol, 7-KC)

• Advantages

• Lysosphingolipids (Lyso-SM-509, lysosphingomyelin)

• Objective, quantitative methodology

• Rapid, practical and cost-effective*

• Bile acids (3β,5α,6β-trihydroxycholanic acid)

• Biomaterials easily accessible and transportable

• Disadvantages

• Available for relatively few ultra-rare IEMs

• Requires that disease of interest is already in differential diagnosis

• Patient heterogeneity can present a hurdle, with possible false-negatives/positives

Genetic analysis

• Single-gene sequencing

• Advantages

• Gene panel (e.g., ataxia panel)

• Objective screening data

• WES

• No requirement for differential diagnosis

• WGS

• Can provide information on diseases not in differential diagnosis

• Might indicate alternate molecular diagnosis

• Disadvantages

• Not yet widely available without appreciable costs

• Limited information on pathogenicity of unique gene variants (potential false negatives and false positives)

• Challenging management of VUS in symptomatic patients without biochemical marker findings

• Management of incidental findings

Clinical assessment

• Multi-disciplinary assessment of clinical manifestations

• Advantages

• Widespread availability of professionals capable of carrying out clinical assessment

• Differential diagnosis

• Assessment of clinical picture from patient files

• Traditional approach set up in healthcare systems

• Disadvantages

• Can be time consuming

• NP-C SI

• Require multiple inter-disciplinary referrals

• Variation in quality: assessments not based on validated clinical tools require IEM expert knowledge to detect disease

• Non detection of atypical/non-standard or early-stage presentations due to non-specific clinical phenotypes

• Do not deliver diagnosis per se although diagnoses can be confirmed using biomarkers and/or genetic methods