No. A normal urine glycosaminoglycan (GAG) test cannot completely rule out mucopolysaccharidosis (MPS). Although urinary GAG analysis is commonly used as an initial screening investigation, some affected individuals may have normal or borderline results.

This is an important consideration when evaluating children with clinical features suggestive of an inherited metabolic disorder.

Why can a urine test be normal in MPS?
Mucopolysaccharidosis occurs when the body lacks sufficient activity of a specific enzyme required to break down glycosaminoglycans. These substances accumulate in tissues and may also be excreted in urine.

Urinary GAG testing looks for abnormalities in the amount or pattern of these substances. However, the result can be influenced by several factors.
1. Mild or attenuated forms of MPS
Some individuals with milder forms of mucopolysaccharidosis may excrete smaller amounts of GAGs. Their urinary results may fall within the laboratory’s reference range despite having the disorder.

2. Age-related differences
Urinary GAG levels vary with age. Interpretation must therefore take the patient’s age and the laboratory’s reference intervals into account.

3. Limitations of the testing method
Different laboratories use different methods to measure urinary GAGs. Some screening methods measure total GAG concentration, while others identify specific GAG patterns.
A test that measures total GAGs may not detect every affected individual, particularly when the abnormality is subtle.

4. Differences between MPS types
The sensitivity of urinary screening varies between MPS types. Patients with Morquio syndrome (MPS IVA or IVB) and some individuals with attenuated Sanfilippo syndrome (MPS III) may have less pronounced urinary GAG abnormalities.
Therefore, a normal screening result should not automatically end the diagnostic investigation when clinical suspicion remains high.

What should be done if the urine test is normal?
If the clinical features strongly suggest mucopolysaccharidosis, the next step is to consider further investigations rather than relying solely on the screening result.

The evaluation may include:
1. Specific enzyme activity testing
This assesses the activity of the enzyme associated with the suspected MPS type. It is a central investigation for confirming the biochemical diagnosis.

2. Molecular genetic testing
Testing the relevant genes can identify disease-associated variants and help establish the molecular diagnosis. The results should be interpreted alongside enzyme activity and clinical findings.

3. Specialist evaluation
A clinical geneticist or metabolic disease specialist can assess the patient’s symptoms, determine which investigations are appropriate, and consider alternative diagnoses if MPS is not confirmed.

Can genetic testing be performed after a normal urine result?
Yes. When symptoms suggest mucopolysaccharidosis, genetic testing may be appropriate despite a normal urine screening result. However, genetic testing should be selected and interpreted carefully because not every detected variant is disease-causing.
A variant of uncertain significance cannot independently confirm MPS, and a negative genetic result may require further evaluation depending on the test’s scope and the patient’s clinical presentation.

Why does early diagnosis matter?
Delaying diagnosis because of a normal screening result may postpone appropriate treatment and monitoring. Some MPS-specific treatments are most beneficial when started early, before significant complications develop.