The key to diagnosis is clinical suspicion of the disease. Clinical and laboratory diagnosis should be coordinated—once there is clinical suspicion, laboratory diagnosis is usually straightforward. These are multi-system disorders with few signature manifestations which occur among all ethnicities. The carrier rate among Ashkenazi Jews is about 1 in 15 for GD, 1 in 30 for Tay-Sachs, and 1 in 80 to 100 for Niemann-Pick type A.
Accurate clinical history and physical examination are crucial. The history must include a detailed family history and inquiry about consanguinity within the pedigree. Delay in diagnosis is a common problem and often leads to inappropriate investigations, delay in instituting specific treatment, and avoidable complications. A particular pitfall in diagnosis for pediatric and adult hematologists is that LSDs may have features that are shared by hematologic conditions (e.g., anemia, thrombocytopenia, hepatosplenomegaly, bone lesions). A survey of hematologists in the USA who have a clinical history of classic type 1 GD revealed that the overwhelming majority were initially considered to have a hematologic malignancy. The majority of Gaucher patients make their initial presentation to a hematologist and report a delay in diagnosis.
Screening Programs
Screening programs aim to reduce diagnostic delay by institution of coordinated community diagnosis. Newborn screening (NBS) is already performed worldwide for certain metabolic diseases where early intervention is widely available and effective at preventing long term complications (e.g., dried blood spot [DBS] tests for phenylketonuria and hypothyroidism). Many states in the USA and countries in Europe and worldwide have embarked on these programs. Most of the laboratories engaged in LSD NBS carry out first-tier testing by measurement of lysosomal enzymatic activities in DBS on NBS cards using either tandem mass spectrometry (MS/MS) or fluorimetry (either digital microfluidics [DMF-F] or a standard plate reader). Confirmatory enzymatic and DNA analysis is performed on positive samples. An emerging problem is that the programs are generally associated with a significant false positive rate. Furthermore, for some conditions (e.g., Fabry) neonatal screening based on gene sequencing has revealed prevalence rates which are far higher than previously sus pected (e.g., 1:3200 for Fabry), but the majority of detected variants are of uncertain significance (GVUS), raising important ethical and procedural issues.
Targeted screening is the approach whereby clinical algorithms are used to identify subjects and family members considered to be at increased risk of suffering an LSD who can then be offered accelerated laboratory diagnosis. Such clinical algorithms have been devised for GD based on expert guidance. A multi-national multi-center targeted screening study of 500 subjects with splenomegaly attending hematologists in Europe yielded a diagnosis of GD among 15 subjects. Other examples of successful targeted screening programs include screening for Fabry disease among subjects with left ventricular hypertrophy and premature “cryptogenic” stroke.
Laboratory Diagnosis
DBS samples are increasingly used, replacing anticoagulated whole blood and, in some cases urine, and referred to specialty laboratories so that common pitfalls in laboratory diagnosis are avoided. Full clinical details and family history should be provided. Tissue biopsy is not indicated. Blood film morphology may give important clues (see below). Specific enzyme activity will then be measured in white blood cells, lymphocytes (such as T cells), plasma or DBS. Cultured fibroblasts are required to measure the activity of certain enzymes, for example, neuraminidase. Mass spectrometric assays allow simultaneous analysis of activity of several enzymes from DBS. Positive and negative controls are often included. Deficient activity in a clinically normal individual may also indicate “pseudodeficiency” due to the presence of a polymorphism—for example, as is commonly found with arylsulfatase A. A false-negative result may indicate a laboratory error but could rarely occur due to deficiency of a sphingolipid activator protein (SAP, e.g., the GM-2 activator protein or saposins A–D). Enzymatic activity from a heterozygous female will be normal or only marginally reduced if the condition is X-linked. Confirmatory DNA analysis should always be performed Laboratory diagnosis should also include confirmatory tests, for example, measurement of substrate level and relevant biomarkers.
Baseline and Sequential Assessment
A genetic counselor can help with the interpretation of the results, the organization of pedigree analysis, and guidance with respect to treatment. A biomarker is a laboratory analyte that reflects the presence and/or extent of a biological process, and should be directly linked to the clinical manifestations and treatment outcome. Quantifiable biomarkers, which are specific, sensitive, reliable, and reproducible, are of great value in the management of several LSDs.
The diversity of clinical manifestations means that multidisciplinary clinical assessment should be conducted to document structure, function, and patient-reported elements at diagnosis and at specified time intervals thereafter, irrespective of whether or not specific treatments are applied. Specific therapy should improve the disease manifestations, yet in many LSDs the impact of the intervention is much more limited, and may only amount to structural or functional reduction in the natural rate of decline. Reliable data on untreated individuals is crucial. Regulatory and funding authorities increasingly regard patient-reported outcome (PRO) as good measures reflecting the quality of life (QOL) and the overlay of psycho logical, social, societal concerns and stressors which are unique to LSD patients.
