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Laboratory Tests

The laboratory tests for diagnosing MPS and identifying carriers are described below.

Urine GAG Analysis—Presumptive Diagnosis

MPS disorders are associated with elevated urinary GAG levels. Laboratory tests are available to detect elevated levels of GAG. Measurement of overall GAG level, or levels of specific GAGs such as dermatan sulfate (the predominant substrate that accumulates in MPS VI) provides preliminary diagnosis and may be useful in guiding specific enzyme assays. Even when elevated GAG levels are detected, urine GAG analysis by itself cannot always establish the specific type of MPS disease present. Nor are these assays immune to the problems of false positive and false negative results.1,2

Enzyme Assays—Definitive Diagnosis

Once the suspicion of MPS VI has been raised by elevated urinary GAG levels, an enzyme assay provides the definitive diagnosis. Enzyme assays measure the activity of the specific lysosomal enzyme in question in either peripheral blood leukocytes or cultured fibroblasts.2

A result showing abnormally low levels of arylsulfatase B activity, together with normal activity levels of other lysosomal GAG sulfatase enzymes, indicates MPS VI. It is important that activity levels of the other GAG sulfatases be assayed along with arylsulfatase B, in order to distinguish MPS VI from mutliple sulfatase deficiency, a related lysosomal storage disorder.2,5

Only qualified biochemical laboratories should perform these specialized assays.

Prenatal Diagnosis

Prenatal diagnosis is available for the MPS disorders, including MPS VI. Measurement of the amount of enzyme activity can be performed using cells isolated from amniotic fluid or chorionic villus biopsies. Amniotic fluid analysis provides more accurate results than chorionic villus sampling.2,6

Prenatal diagnosis is used when there is a risk of MPS in a particular pregnancy—for example, when both parents are known to be carriers or when a close relative has been diagnosed with an MPS disorder.2

Carrier Testing and Mutation Analysis

For unaffected adult siblings of patients with MPS VI, carrier testing can provide information regarding the risk of having an affected child.2

Analyses of mutations or genetic linkage can provide exact information on a potential carrier's status. Since MPS VI can be caused by any of a large number of different mutations in the arylsulfatase B gene, the exact mutations being tested for should ideally be known ahead of time (most MPS VI patients are compound heterozygotes—carrying 2 different mutant alleles—and so knowledge of both alleles is required in order to determine carrier status).2,6,7

When mutation analysis cannot be used—for example, when the identity of the mutation is unknown—MPS carrier status has been evaluated by assaying activity levels of the relevant enzyme (arylsulfatase B in the case of MPS VI). However, normal and abnormal ranges of enzyme activity levels are broad and overlapping, often making this method unreliable.2

Locating a Specialized Testing Laboratory

The assays and tests described above are highly specialized and are typically performed by a laboratory experienced in MPS-related diagnostics. A medical geneticist or metabolic specialist experienced in the diagnosis of MPS disorders will have knowledge of appropriate diagnostic resources.

  1. Paterson DE, Harper G, Weston HJ, Mattingley J. Maroteaux-Lamy syndrome, mild form—MPS vi b. Br J Radiol. 1982;55:805-812.
  2. Neufeld EF, Muenzer J. The mucopolysaccharidoses. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. Vol 3. 8th ed. New York, NY: McGraw- Hill; 2001:3421-3452.
  3. TKT receives fast track designation for iduronate-2-sulfatase for Hunter syndrome [press release]. Cambridge, MA: Transkaryotic Therapies, Inc.; July 15, 2004. Available at: http://www.tktx.com/newsbureau/press.htm. Accessed December 17, 2004.
  4. Harmatz P, Whitley CB, Waber L, et al. Enzyme replacement therapy in mucopolysaccharidosis VI (Maroteaux-Lamy syndrome). J Pediatr. 2004;144:574-580.
  5. Whitley CB. The mucopolysaccharidoses. In: Beighton P, ed. McKusick's Heritable Disorders of Connective Tissue. 5th ed. St Louis, Mo: Mosby; 1993:367-499.
  6. Muenzer J. The mucopolysaccharidoses: a heterogeneous group of disorders with variable pediatric presentations. J Pediatr. 2004;144(5 Suppl):S27-S34. Review.
  7. Kakkis ED, Neufeld EF. The mucopolysaccharidoses. In: Berg BO, ed. Principles of Child Neurology. New York, NY: McGraw-Hill; 1996:1141-1165.
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