Apt Test in Newborns with Suspected Gastrointestinal Bleeding
Primary Purpose
The Apt test distinguishes swallowed maternal blood from true neonatal gastrointestinal bleeding by differentiating fetal hemoglobin (HbF) from adult hemoglobin based on their differential resistance to alkaline denaturation. 1
Clinical Context and Indication
Swallowed maternal blood—either during delivery or from cracked nipples during breastfeeding—is the most common cause of suspected gastrointestinal bleeding in neonates, making the Apt test essential for avoiding unnecessary invasive workups 1
The test is specifically indicated when neonates present with bloody stools (melena neonatorum) or hematemesis to determine whether the blood originates from the infant's GI tract or represents swallowed maternal blood 2
This distinction has critical implications for management: maternal blood requires no intervention beyond reassurance, while true neonatal GI bleeding may require urgent evaluation for serious conditions like necrotizing enterocolitis, coagulopathy, or anatomic abnormalities 1
Test Mechanism
The Apt-Downey test exploits the fact that fetal hemoglobin is resistant to alkaline denaturation, while adult hemoglobin converts to alkaline globin hematin (appearing brown) when mixed with sodium hydroxide 1
When the stool sample containing blood is mixed with alkali, fetal hemoglobin remains pink/red, while maternal hemoglobin turns yellow-brown, providing rapid visual differentiation 2
Modern spectrophotometric modifications measure the absorbance ratio at 576 nm before and after alkaline treatment, with the ratio linearly proportional to the percentage of HbF present (ratio = 0.01 × %HbF + 0.0045) 3
Practical Performance
The test can be completed in approximately 4-7 minutes for the traditional Apt or Loendersloot methods, or 5 minutes for the Ogita modification, making it ideal for stat diagnosis 4
The spectrophotometric version provides quantitative results with high precision (r = 0.9971) and accuracy when compared to radial immunodiffusion (r = 0.91), and is free from bilirubin interference 3
The test requires minimal equipment and can be performed at the bedside or in a basic laboratory, making it practical for routine use in labor and delivery or neonatal units 4
Critical Caveats
The Apt test detects fetal hemoglobin concentrations as low as 20-60% depending on the method used, meaning small amounts of fetal blood mixed with large volumes of maternal blood may yield false-negative results 4
The test does not identify the specific source or severity of neonatal GI bleeding—it only confirms that bleeding is of fetal origin, necessitating further diagnostic evaluation when positive 2
While highly specific for differentiating maternal from fetal blood, the test should not delay resuscitation or stabilization in hemodynamically unstable neonates with significant bleeding 1