What is the most appropriate next investigation for an 18-month-old child who develops jaundice within one day?

Most Appropriate Next Investigation for an 18-Month-Old Child with Acute Jaundice

The most appropriate next investigation is measurement of total and direct (or conjugated) bilirubin levels, followed by a complete blood count, liver function tests, and urinalysis with urine culture. 1, 2

Critical Context: This is NOT Neonatal Jaundice

• An 18-month-old child developing jaundice within one day represents pathologic jaundice requiring urgent evaluation, as this is far beyond the neonatal period where physiological jaundice occurs 1
• The acute onset (within one day) in a toddler suggests serious underlying pathology including hemolysis, hepatitis, sepsis, biliary obstruction, or metabolic disease 1

Initial Laboratory Workup

Immediate blood tests should include:

• Total and direct (conjugated) bilirubin to differentiate obstructive from non-obstructive causes 1, 2
• Complete blood count with peripheral smear to evaluate for hemolysis, infection, or hematologic disorders 1, 2
• Liver function tests including AST, ALT, alkaline phosphatase, GGT, and albumin to assess hepatocellular injury versus cholestasis 1, 3
• Prothrombin time/INR to evaluate synthetic liver function 3

Additional critical tests:

• Blood type and Coombs test if hemolysis is suspected 1, 2
• Reticulocyte count and G6PD level if anemia or hemolysis is present 1, 2, 4
• Urinalysis and urine culture to evaluate for urinary tract infection, which can cause jaundice in young children 1, 5

Fractionated Bilirubin Guides Further Workup

If conjugated (direct) hyperbilirubinemia is present (>20% of total bilirubin):

• This indicates cholestasis or biliary obstruction and requires urgent evaluation for biliary atresia (though less likely at 18 months), choledochal cyst, hepatitis, or metabolic disease 1, 6
• Abdominal ultrasound should be performed to evaluate the biliary tree and liver parenchyma 1
• Evaluate for sepsis with blood cultures if clinically indicated 1, 2

If unconjugated (indirect) hyperbilirubinemia predominates:

• Consider hemolytic causes (G6PD deficiency, autoimmune hemolytic anemia, blood group incompatibility) 1, 4
• Evaluate for sepsis, as this is a common cause of jaundice in children 1
• Consider Gilbert syndrome or other inherited disorders of bilirubin metabolism 1

Imaging Considerations

• Abdominal ultrasound is the initial imaging modality of choice if biliary obstruction or hepatobiliary disease is suspected based on conjugated hyperbilirubinemia or clinical findings 1
• Ultrasound can detect biliary dilation, gallstones, liver masses, and assess liver parenchyma 1

Critical Pitfalls to Avoid

• Never rely on visual estimation of jaundice severity—always obtain objective bilirubin measurements, particularly in children with darker skin pigmentation 1, 2
• Do not delay evaluation—acute jaundice in an 18-month-old is pathologic until proven otherwise and requires same-day assessment 4
• Always measure direct/conjugated bilirubin—failure to do so may miss serious cholestatic liver disease requiring urgent intervention 1, 6, 5
• Consider sepsis as a leading cause—in one U.S. study, sepsis was the most common etiology of new-onset jaundice (22% of cases) 1
• Evaluate stool and urine color—pale stools or dark urine suggest conjugated hyperbilirubinemia and cholestasis 4, 5