The Hydrostatic Test (Lung Flotation Test)
Forensic Pathology: Determining Live Birth in Neonaticide Cases
1. Clinical Purpose
The hydrostatic test is used to determine if a neonate was born alive (breathed air) or was stillborn. It relies on the principle of buoyancy—specifically, the difference in density between non-aerated lung tissue and lungs that have been inflated with air.
2. The Procedure
- Step 1: The thoracic cavity is opened, and the heart and lungs are removed as a unit.
- Step 2: The unit is placed in a container of water. If it floats, it suggests the presence of air (positive test).
- Step 3: The lungs are separated from the heart and placed in water individually.
- Step 4: Finally, the lung lobes are cut into small pieces and placed in water. If pieces float, the test is confirmed as positive.
3. Forensic Interpretation
| Result | Interpretation |
|---|---|
| Positive | Lungs float (implies air entry/breathing). |
| Negative | Lungs sink (implies stillborn). |
4. Forensic Limitations & Pitfalls
- False Positives: Decomposition (putrefaction) produces gases (
,
) that can cause lungs to float even if the infant never breathed.
- False Negatives: Respiratory distress syndrome (RDS) or amniotic fluid aspiration can prevent enough air entry to make the lungs float, even in a live-born infant.
- Resuscitation: Artificial ventilation (e.g., CPR) can inflate the lungs, leading to a false positive result for a “born alive” status.