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The second step in the evaluation is to identify which general laboratory testing to perform. This step is critical not only to obtain laboratory data to assist in the diagnosis but also, and potentially of greater immediate importance, to identify special areas requiring treatment. For example, in urea cycle defects, organic acid disorders, and fatty acid oxidation defects, it is important to obtain the blood ammonia level to diagnose hyperammonemia. Definitive diagnosis of one of these disorders typically requires special biochemical diagnostic testing, which is not performed in many hospital laboratories. However, initiating this testing following an ED visit or subsequent hospitalization can greatly facilitate the diagnosis of one of these conditions and assist in the patient's medical management.
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General laboratory testing can assist in determining if an IEM is present. Table 79-3 lists some of the general laboratory testing performed in this clinical setting. Testing performed in most situations would include glucose levels, evaluation of acid/base status, urine analysis including urine ketones, complete blood count, and evaluation of renal and liver functions. Additional testing may be indicated. For example, lactate level to assess for lactic acidosis and ammonia level for hyperammonemia is indicated in cases of acidosis or altered mental status, respectively. If evidence of myopathy, muscle weakness, or myalgia is present, especially with an elevation in AST/ALT, obtain a creatine kinase (CK) level to evaluate for a myopathic process and rhabdomyolysis. Elevated CK is present in some fatty acid oxidation deficiencies (FAOD).1,2,7,8 The presence of blood on urinalysis with normal urine RBC count in this setting may be indicative of myoglobinuria. Blood myoglobin levels can be obtained in some hospital laboratories to evaluate this finding further.
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Initial laboratory testing is usually guided by the problem present at the time of presentation to the ED. In some situations, the order of testing may be influenced by both the clinical state of the patient and results of initial testing. Figure 79-1 outlines an approach for evaluation of hypoglycemia in a case of a suspected IEM. Hypoglycemia with negative ketone levels may indicate the presence of an FAOD, including medium chain acyl CoA dehydrogenase deficiency (MCADD), while the presence of acidosis, hypoglycemia, and ketosis is more suggestive of an organic acid disorder. Hypoglycemia is also reviewed in Chapter 76, Diabetes Mellitus and Hypoglycemia.
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It is important to recognize that complicating factors present at the time of the patient's initial presentation may impact test results and their interpretation. A severely dehydrated child with poor perfusion may also have a secondary lactic acidosis. The patient's clinical course, including laboratory results and response to treatment, usually allows for a determination as to whether the abnormalities are secondary or the consequence of an IEM. Initial treatment of identified laboratory findings usually follows general recommendations.
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Table 79-4 lists some of the specialized biochemical genetic testing considered in the evaluation of IEM. Specific testing to be performed is often determined in consultation with the biochemical/clinical geneticist or other metabolic specialist. In many situations, biochemical genetic testing performed at the time of an acute illness can evaluate for specific abnormalities helpful in the IEM diagnostic evaluation. In some situations, diagnostic test results may only be detectable during an acute illness.
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