Pediatric emergency medicine trisk 609
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Acidosis. The differential diagnosis of metabolic acidosis is broad (see Chapter 100 Renal and
Electrolyte Emergencies ) but IEMs must be considered in patients with unexplained or greater-thanexpected metabolic acidosis. Clinical manifestations of acidosis are nonspecific and include vomiting and
tachypnea. Primary metabolic acidosis is diagnosed by low pH, low PCO 2 , and low bicarbonate. An
elevated anion gap (>16 mmol/L), which is characteristic of acute metabolic crisis with many IEMs,
helps distinguish among causes of metabolic acidosis. An elevated anion gap with a normal chloride
usually reflects excess acid production, most often of lactate, ketone bodies, and/or other organic acids.
Organic acidemias are characterized by metabolic acidosis, usually severe, with marked ketonuria, with
or without hyperammonemia or hypoglycemia. In neonates believed to have pyloric stenosis, the
diagnosis of IEM, particularly an organic acidemia, should be considered if the patient has metabolic
acidosis rather than metabolic alkalosis. Fatty acid oxidation disorders may also present with metabolic
acidosis, but usually with hypoglycemia and absent ketones or hypoketosis. The IEMs in which a primary
lactic acidosis is the cause of the metabolic acidosis include disorders of gluconeogenesis and
mitochondrial disorders of oxidation. In patients with metabolic acidosis, concentration of serum
ammonia and glucose, and presence or absence of urine ketones and reducing substances will also help
direct further metabolic workup. Plasma amino acids, acylcarnitines and urine organic acids, and
acylglycines should be measured. Measurement of serum lactate, pyruvate, ketones, and organic acids
may also be helpful.
TABLE 95.5
SECONDARY TESTS
Test
Laboratory abnormality
metabolic diseases a
Indications, comments
Aminoacidopathies
Organic acidemias
Tandem mass spectrometry, requires
minimum 1 mL blood, 3 mL ideal b ,
heparin, or EDTA tube
Obtain if metabolic catastrophe, neurologic,
cardiac, GI/hepatic, musculoskeletal,
psychiatric symptoms suggestive of
possible IEM, metabolic acidosis, elevated
anion gap, hypoglycemia, inappropriate
ketonuria, hyperammonemia
Blood
Amino acids—
quantitative (plasma
or serum)
Urea cycle defects
Mitochondrial disorders
Acylcarnitine profile
(plasma or serum)
Organic acidemias
Fatty acid oxidation defects
Mitochondrial disorders
Primary carnitine deficiency
Lactate, pyruvate
Disorders of carbohydrate
(deproteinized blood)
utilization
Mitochondrial disorders
Carnitine deficiency may be due to primary
defect in carnitine or carnitine transporter,
or secondary due to organic acidemia or
fatty acid oxidation defect; can also occur
in normal children during dehydration
Free and total carnitine may also be helpful if
carnitine deficiency is suspected
Samples must be free flow, deproteinized at
bedside—1 mL into tubes with 2-mL
perchloric or trichloroacetic acid, transport
on ice
Evaluate lactate, pyruvate, and ratio
Lactate also increased in patient with
hypoxia, poor perfusion, sepsis
Urine
Organic acids
Aminoacidopathies
Organic acidemias
Fatty acid oxidation defects
Mitochondrial disorders
Peroxisomal disorders
Acylglycines
Organic acidemias
Fatty acid oxidation defects
Orotic acid
Urea cycle defects (ornithine
transcarbamylase
deficiency)
Urine best source for organic acids, minimum
2–5 mL, 10–20 mL ideal without
preservative c
Obtain if metabolic catastrophe, neurologic,
cardiac, GI/hepatic, musculoskeletal,
psychiatric, symptoms suggestive of
possible IEM, metabolic acidosis, elevated
anion gap, hypoglycemia, inappropriate
ketonuria, hyperammonemia
Should be performed only in conjunction
with serum or plasma carnitines, minimum
2–5 mL without preservative c
Send if hyperammonemia, minimum 1 mL
without preservative
Cerebrospinal fluid
Glucose, protein, lactate, Aminoacidopathies
pyruvate, glycine,
Organic acidemias
serine, alanine,
1–4 mL, freeze −20°C or −70°C
organic acids,
neurotransmitters,
folate, pterins, other
disease-specific
metabolites
Mitochondrial disorders
Nonketotic hyperglycinemia
Neurotransmitter disorders
a Within
disease categories, not all diseases have the laboratory abnormality. In disorders of protein metabolism, carbohydrate metabolism,
and fatty acid oxidation defects and abnormality may be present only during acute crisis.
b Samples, quantities required, collection method, preparation, and storage are institution dependent. Tandem mass spectrometry measures
amino acids and acylcarnitines, derived from carnitine, which combines with acyl-CoA derived from fatty acids and organic acids (which
may have been derived from amino acids). Tandem mass spectrometry may be used as a screen for aminoacidopathies, organic acidemias,
and fatty acid oxidation defects. Confirmation of diagnosis usually requires further testing, including plasma amino acids, urinary organic
acids, histologic examination, DNA analysis, and enzyme and/or biochemical assays.
c Total minimum is 4 mL for organic acids and acylglycines.
EDTA, ethylenediaminetetraacetic acid; GI, gastrointestinal; IEM, inborn error of metabolism.
Hyperammonemia. Ammonia is an intermediary in the catabolism of nitrogen-containing compounds,
particularly amino acids. Normal ammonia levels are less than 100 μg/dL in neonates and less than 80
μg/dL beyond the neonatal period. Elevated ammonia is neurotoxic and immediate treatment should
begin pending confirmation of a specific diagnosis. Early manifestations of hyperammonemia are
anorexia and irritability, followed by rapid progression to vomiting, lethargy, seizures, coma, and death in
hours. Marked hyperammonemia causes brainstem dysfunction leading to deep rapid breathing and
resulting in respiratory alkalosis, a hallmark of urea cycle disorders. Cerebral edema and intracranial
hemorrhage are also common. Children and adolescents may report headache, abdominal pain, and
fatigue. Some patients with chronic hyperammonemia adapt to their elevated ammonia level and may
appear to have no overt symptoms despite elevated ammonia.
