A diagnosis is made by measuring the enzymatic activity of alpha--mannosidase in white blood cells. If there is a decreased level of the enzyme in comparison to standard levels, a diagnosis can be made. It is thought that this disorder might be under-diagnosed for a few different reasons—the diagnosis is often made late in the disease's progression, symptoms are often mild, or the biochemical diagnosis does not yield conclusive results.

In terms of the diagnosis for glycogen storage disease type III, the following tests/exams are carried out to determine if the individual has the condition:

- Biopsy (muscle or liver)

- CBC

- Ultrasound

- DNA mutation analysis (helps ascertain GSD III subtype)

The life expectancy in alpha-mannosidosis is highly variable. Individuals with early onset severe disease often do not survive beyond childhood, whereas those with milder disorders may survive well into adult life.

Diagnosis: A special urine test is available to check for any partially broken-down-sugars. If they are present, a skin or blood sample will be taken to test for below-normal amounts of alpha-fucosidase.

- Fucosidosis is an autosomal recessive disorder, which means that both parents have to have the mutation and pass it on to the child. When both parents have the mutation, there is a 25% chance of each child having fucosidosis.

The differential diagnosis of glycogen storage disease type III includes GSD I, GSD IX and GSD VI. This however does not mean other glycogen storage diseases should not be distinguished as well.

The usual initial investigations include chest X ray, electrocardiogram and echocardiography. Typical findings are those of an enlarged heart with non specific conduction defects. Biochemical investigations include serum creatine kinase (typically increased 10 fold) with lesser elevations of the serum aldolase, aspartate transaminase, alanine transaminase and lactic dehydrogenase. Diagnosis is made by estimating the acid alpha glucosidase activity in either skin biopsy (fibroblasts), muscle biopsy (muscle cells) or in white blood cells. The choice of sample depends on the facilities available at the diagnostic laboratory.

In the late onset form, the findings on investigation are similar to those of the infantile form with the caveat that the creatinine kinases may be normal in some cases. The diagnosis is by estimation of the enzyme activity in a suitable sample.

On May 17, 2013 the Secretary's Discretionary Advisory Committee on Heritable Diseases in Newborns and Children (DACHDNC) approved a recommendation to the Secretary of Health and Human Services to add Pompe to the Recommended Uniform Screening Panel (RUSP). The HHS secretary must first approve the recommendation before the disease is formally added to the panel.

Type 2 appears when a child is around 18 months of age and in considered milder than Type 1 but still severe. Symptoms include:

- Symptoms similar to Type 1 but milder and progress more slowly.

Diagnostic methods

Diagnosis is based mainly on clinical findings and laboratory test results. Plasma concentrations of ammonia (>150 µmol/L) and citrulline (200-300 µmol/L) are elevated. Elevated levels of argininosuccinic acid (5-110 µmol/L) in the plasma or urine are diagnostic. Molecular genetic testing confirms diagnosis. Newborn screening for ASA is available in the U.S. and parts of Australia, and is considered in several European countries<

In terms of diagnosis of Fukuyama congenital muscular dystrophy, serum creatine kinase concentration and muscle biopsies can be obtained to help determine if the individual has FMCD. FKTN molecular genetic testing is used to determine a mutation in the FKTN gene after a serum creatine kinase concentration, muscle biopsies, and/or MRI imaging have presented abnormalities indicative of FCMD, the presence of the symptoms indicates Fukuyama congenital muscular dystrophy. The available genetic test include:

- Linkage analysis

- Deletion analysis

- Sequence analysis - exons

- Sequence analysis - entire coding region

Amniocentesis or chorionic villus sampling can be used to screen for the disease before birth. After birth, urine tests, along with blood tests and skin biopsies can be used to diagnose Schindler disease. Genetic testing is also always an option, since different forms of Schindler disease have been mapped to the same gene on chromosome 22; though different changes (mutations) of this gene are responsible for the infantile- and adult-onset forms of the disease.

The majority of patients is initially screened by enzyme assay, which is the most efficient method to arrive at a definitive diagnosis. In some families where the disease-causing mutations are known and in certain genetic isolates, mutation analysis may be performed. In addition, after a diagnosis is made by biochemical means, mutation analysis may be performed for certain disorders.

Infants with Schindler disease tend to die within 4 years of birth, therefore, treatment for this form of the disease is mostly palliative. However, Type II Schindler disease, with its late onset of symptoms, is not characterized by neurological degeneration. There is no known cure for Schindler disease, but bone marrow transplants have been trialed, as they have been successful in curing other glycoprotein disorders.

Diagnosis often can be made through clinical examination and urine tests (excess mucopolysaccharides are excreted in the urine). Enzyme assays (testing a variety of cells or body fluids in culture for enzyme deficiency) are also used to provide definitive diagnosis of one of the mucopolysaccharidoses. Prenatal diagnosis using amniocentesis and chorionic villus sampling can verify if a fetus either carries a copy of the defective gene or is affected with the disorder. Genetic counseling can help parents who have a family history of the mucopolysaccharidoses determine if they are carrying the mutated gene that causes the disorders.

The treatment of 2-Hydroxyglutaric aciduria is based on seizure control, the prognosis depends on how severe the condition is.

Due to the rarity of the disease, it is hard to estimate mortality rates or life expectancy. One 2003 study which followed 88 cases receiving two different kinds of treatment found that very few persons lived beyond age 20 and none beyond age 30.

There is a deficiency of malate in patients because fumarase enzyme can't convert fumarate into it therefore treatment is with oral malic acid which will allow the krebs cycle to continue, and eventually make ATP.

There are exceptions, but levels of alpha-glucosidase determines the type of GSD II an individual may have. More alpha glucosidase present in the individuals muscles means symptoms occur later in life and progress more slowly. GSD II is broadly divided into two onset forms based on the age symptoms occur.

Infantile-onset form is usually diagnosed at 4–8 months; muscles appear normal but are limp and weak preventing them from lifting their head or rolling over. As the disease progresses heart muscles thicken and progressively fail. Without treatment death usually occurs due to heart failure and respiratory weakness.

Late or later onset form occurs later than one to two years and progresses more slowly than Infantile-onset form. One of the first symptoms is a progressive decrease in muscle strength starting with the legs and moving to smaller muscles in the trunk and arms, such as the diaphragm and other muscles required for breathing. Respiratory failure is the most common cause of death. Enlargement of the heart muscles and rhythm disturbances are not significant features but do occur in some cases.

A 2006 study of 279 patients found that of those with symptoms (185, 66%), 95% had suffered an encephalopathic crises usually with following brain damage. Of the persons in the study, 49 children died and the median age of death was 6.6 years. A Kaplan-Meier analysis of the data estimated that about 50% of symptomatic cases would die by the age of 25.

Less than 20 patients with MGA type I have been reported in the literature (Mol Genet Metab. 2011 Nov;104(3):410-3. Epub 2011 Jul 26.)

Fukuyama congenital muscular dystrophy has a poor prognosis. Most children with FCMD reach a maximum mobility at sitting upright and sliding. Due to the compounded effects of continually worsening heart problems, impaired mental development, problems swallowing and additional complications, children with FCMD rarely live through adolescence, the disorder proves fatal by age 20.

Seven distinct clinical types and numerous subtypes of the mucopolysaccharidoses have been identified. Although each mucopolysaccharidosis (MPS) differs clinically, most patients generally experience a period of normal development followed by a decline in physical and/or mental function. (Note: MPS-V and MPS-VIII are no longer in use as designations for any disease.)

Detection of the disorder is possible with an organic acid analysis of the urine. Patients with SSADH deficiency will excrete high levels of GHB but this can be difficult to measure since GHB has high volatility and may be obscured on gas chromatography or mass spectrometry studies by a high urea peak. Other GABA metabolites can also be identified in urine such as glycine. Finally, succinic semialdehyde dehydrogenase levels can be measured in cultured leukocytes of the patient. This occurs due to the accumulation of 4,5-dihydroxyhexanoic acid which is normally undetectable in mammalian tissues but is characteristic of SSADH deficiency. This agent can eventually compromise the pathways of fatty acid, glycine, and pyruvate metabolism, and then become detectable in patients' leukocytes. Such enzyme levels can also be compared to non-affected parents and siblings.

Stress caused by infection, fever or other demands on the body may lead to worsening of the signs and symptoms, with only partial recovery.

The symptoms of LSD vary, depending on the particular disorder and other variables such as the age of onset, and can be mild to severe. They can include developmental delay, movement disorders, seizures, dementia, deafness, and/or blindness. Some people with LSDhave enlarged livers (hepatomegaly) and enlarged spleens (splenomegaly), pulmonary and cardiac problems, and bones that grow abnormally.

In horses: it has been reported in American Quarter Horses and related breeds.

In cats: the disease has been reported in the Norwegian Forest Cat, where it causes skeletal muscle, heart, and CNS degeneration in animals greater than 5 months old. It has not been associated with cirrhosis or liver failure.