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Interpretive Handbook

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Test 81931 :
Postmortem Screening, Bile and Blood Spots

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Postmortem screening involves acylcarnitine analysis in blood and/or bile specimens to evaluate cases of sudden or unexpected death. Acylcarnitine analysis enables the diagnosis of many disorders of fatty acid oxidation and several organic acidurias, as relevant enzyme deficiencies cause the accumulation of specific acyl-CoAs. Fatty acid oxidation (FAO) plays a major role in energy production during periods of fasting. When the body's supply of glucose is depleted, fatty acids are mobilized from adipose tissue, taken up by the liver and muscles, and oxidized to acetyl-CoA. In the liver, acetyl-CoA is the building block for the synthesis of ketone bodies, which enter the blood stream and provide an alternative substrate for production of energy in other tissues when the supply of glucose is insufficient to maintain a normal level of energy. The acyl groups are conjugated with carnitine to form acylcarnitines, which are measured by tandem mass spectrometry (MS/MS). Diagnostic results are usually characterized by a pattern of significantly elevated acylcarnitine species compared to normal and disease controls.

 

In general, more than 20 inborn errors of metabolism can be identified using this method including FAO disorders and organic acidurias. The major clinical manifestations associated with individual FAO disorders include hypoketotic hypoglycemia, variable degrees of liver disease and/or failure, skeletal myopathy, dilated/hypertrophic cardiomyopathy, and sudden or unexpected death. Organic acidurias also present as acute life-threatening events early in life with metabolic acidosis, increased anion gap, and neurologic distress. Patients with any of these disorders are at risk of developing fatal metabolic decompensations following the acquisition of even common viral infections. Once diagnosed, these disorders can be treated by avoidance of fasting, special diets, and cofactor/vitamin supplementation.

 

Analysis of acylcarnitines in blood and bile spots represents the first level of evaluation of a complete postmortem investigation of a sudden or unexpected death of an individual. Additional confirmatory testing is recommended. The diagnosis of an underlying FAO disorder or organic aciduria allows genetic counseling of the family, including the possible option of future prenatal diagnosis, and testing of at-risk family members of any age.

 

Disorders Detectable by Acylcarnitine Analysis*

Fatty Acid Oxidation Disorders:

-Short-chain acyl-CoA dehydrogenase (SCAD) deficiency

-Medium/Short-chain 3-hydroxyacyl-CoA dehydrogenase (M/SCHAD) deficiency

-Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

-Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency and trifunctional protein deficiency

-Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency

-Carnitine palmitoyl transferase type II (CPT-II) deficiency

-Carnitine-acylcarnitine translocase (CACT) deficiency

-Electron-Transferring Flavoproteins (ETF) deficiency, ETF-dehydrogenase deficiency (multiple acyl-CoA dehydrogenase deficiency: MADD; glutaric acidemia type II)

 

Organic Acid Disorders:

-Glutaryl-CoA dehydrogenase deficiency (glutaric acidemia type I)

-Propionic Acidemia

-Methylmalonic Acidemia

-Isovaleric Acidemia

-3-Hydroxy-3-methylglutaryl-CoA carboxylase deficiency

-3-Methylcrotonyl carboxylase deficiency

-Biotinidase deficiency

-Multiple carboxylase deficiency

-Isobutyryl-CoA dehydrogenase deficiency

-2-Methylbutyryl-CoA dehydrogenase deficiency

-Beta-ketothiolase deficiency

-Malonic aciduria

-Ethylmalonic encephalopathy

 

*Further confirmatory testing is required for most of these conditions because an acylcarnitine profile can be suggestive of more than 1 condition.

See Postmortem Screening Algorithm in Special Instructions. Refer to The Metabolic Autopsy: Postmortem Screening in Cases of Sudden, Unexpected Death, Mayo Medical Laboratories Communique 2003 Sep;28(9) for more information regarding diagnostic strategy.

Useful For Suggests clinical disorders or settings where the test may be helpful

Postmortem evaluation of individuals at any age who died suddenly or unexpectedly; testing is particularly recommended under the following circumstances (risk factors):

-Family history of sudden infant death syndrome or other sudden unexpected deaths at any age

-Family history of Reye syndrome

-Maternal complications of pregnancy (acute fatty liver pregnancy, HELLP syndrome [hemolysis, elevated liver enzymes, and low platelet count])

-Lethargy, vomiting, fasting in the 48 hours prior to death

-Allegation of child abuse (excluding obvious cases of trauma, physical harm)

-Macroscopic findings at autopsy:

   - Fatty infiltration of the liver

   - Dilated or hypertrophic cardiomyopathy

   - Autopsy evidence of infection that routinely would not represent a life-threatening event

Interpretation Provides information to assist in interpretation of the test results

Reports of abnormal acylcarnitine profiles will include an overview of the results and of their significance, a correlation to available clinical information, possible differential diagnoses, recommendations for additional biochemical testing and confirmatory studies (enzyme assay, molecular analysis) as indicated, name and phone number of contacts who may provide these studies at Mayo Clinic or elsewhere, and a phone number to reach one of the laboratory directors in case the referring physician has additional questions.

 

Abnormal results are not always sufficient to conclusively establish a diagnosis of a particular disease. To verify a preliminary diagnosis based on an acylcarnitine analysis, independent biochemical (eg, FAO / Fatty Acid Oxidation Probe Assay, Fibroblast Culture) or molecular genetic analyses are required using additional tissue such as skin fibroblasts from the deceased patient. If not available, molecular genetic analysis of a patient's parents may enable the confirmation of a diagnosis.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Both blood and bile specimens must be collected in order to detect and independently confirm the largest possible number of disorders. However, if only 1 specimen type is available, testing is still beneficial.

 

In cases with a higher level of suspicion due to the recognition of 1 or more risk factors, collection of urine on filter paper and a skin biopsy is also recommended for further testing and enzymatic/molecular studies. Contact the Biochemical Genetic consultant or genetic counselor on call at 800-533-1710 to discuss high risk cases.

 

In comparison to living individuals, profiles of postmortem blood specimens generally show a nonspecific increase of short chain species.

 

Patients with secondary carnitine deficiency may display uninformative acylcarnitine profiles in blood, but not in bile.

 

Several fatty acid oxidation disorders are not associated with abnormal acylcarnitine profiles (eg, carnitine palmitoyltransferase I (CPT I) deficiency, 3-hydroxy-3-methylglutaryl CoA synthase (HMG-CoA synthase) deficiency) and will not be detected.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Quantitative results are compared to a constantly updated range which corresponds to the 5 to 95 percentile interval of all postmortem cases analyzed in our laboratory.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Rinaldo P, Matern D, Bennet BJ: Fatty acid oxidation disorders. Ann Rev Physiol 2002;64:477-502

2. Rashed MS, Ozand PT, Bennett MJ, et al: Inborn errors of metabolism diagnosed in sudden death cases by acylcarnitine analysis of postmortem bile. Clin Chem 1995;41:1109-1114

3. Chace DH, DiPerna JC, Mitchell BL, et al: Electrospray tandem mass spectrometry for analysis of acylcarnitines in dried postmortem blood specimens collected at autopsy from infants with unexplained cause of death. Clin Chem 2001;47:1166-1182

4. Pryce JW, Weber MA, Heales S, et al: Tandem mass spectrometry findings at autopsy for detection of metabolic disease in infant deaths: postmortem changes and confounding factors. J Clin Pathol 2011;64:1005-1009


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