The Apt Test: A Diagnostic Tool for Differentiating Fetal from Maternal Blood
The Apt test is a simple, rapid diagnostic test used to differentiate fetal hemoglobin from maternal hemoglobin in blood samples, particularly useful in determining the source of blood in neonatal gastrointestinal bleeding.
Principle and Methodology
The Apt test (also called Apt-Downey test) works based on the principle that fetal hemoglobin is resistant to alkali denaturation, while adult hemoglobin is not. The test involves:
- Sample preparation: Mix the blood-containing specimen with water to create hemolysis
- Alkali addition: Add sodium hydroxide (NaOH) solution to the mixture
- Observation: Watch for color change over 1-2 minutes
Results Interpretation:
- Pink/red color: Indicates fetal hemoglobin (resistant to alkali denaturation)
- Brown/yellow-brown color: Indicates maternal hemoglobin (denatures in alkali)
Clinical Applications
1. Neonatal Gastrointestinal Bleeding
- Primary use: Differentiating between swallowed maternal blood and true GI bleeding in neonates 1, 2
- Clinical context: When a neonate presents with hematemesis or melena, the most common cause is swallowed maternal blood (from delivery or cracked nipples during breastfeeding) rather than true GI pathology 2
- Clinical impact: Prevents unnecessary invasive investigations and interventions
2. Prenatal Diagnosis
- Cordocentesis verification: Confirms fetal origin of blood samples obtained during genetic cordocentesis 3, 4
- Speed advantage: Provides results within seconds, allowing immediate confirmation of proper sampling 3
- Limitation: May yield a "fetal blood" result even with some maternal contamination 4
3. Obstetric Emergencies
- Vasa previa bleeding: Can help identify fetal bleeding in suspected vasa previa cases 5
- Comparison with alternatives: The Ogita test (a modified alkaline denaturation test) has been found to be more sensitive, detecting fetal blood at concentrations as low as 20%, while the Apt test requires approximately 60% fetal blood for clear positivity 5
Advantages and Limitations
Advantages:
- Quick (results in seconds to minutes)
- Inexpensive
- Simple to perform at bedside
- Does not require specialized equipment
Limitations:
- Less sensitive than specialized tests like Kleihauer-Betke test or hemoglobin electrophoresis
- May not detect small amounts of fetal blood mixed with maternal blood
- Requires adequate sample preparation
Clinical Pearls
- The test is most valuable in the emergency setting when rapid differentiation between maternal and fetal blood is needed
- A spectrophotometric modification of the Apt test has been developed to improve accuracy 1
- When maternal contamination is suspected in prenatal diagnostic samples, additional cytogenetic analysis (counting at least 30 metaphases from different slides) should be performed 4
- For higher sensitivity in detecting small amounts of fetal blood, the Kleihauer-Betke test (which can detect as little as 0.01% fetal blood) may be preferred 5
In clinical practice, the Apt test remains a valuable first-line diagnostic tool when needing to quickly determine if blood in a neonate's vomit or stool is of maternal or fetal origin, potentially avoiding unnecessary interventions and parental anxiety.