A caesium test measures the concentration of the heavy metal caesium in biological samples such as blood or urine. While naturally occurring stable caesium poses minimal health risks, radioactive isotopes like caesium-137 can be released through nuclear accidents or industrial incidents, leading to internal contamination and radiation exposure. This diagnostic test is primarily used in emergency toxicology, occupational surveillance, and environmental medicine to detect exposure, quantify body burden, and guide mitigation or decorporation therapies, such as Prussian blue administration. Clinicians interpret test results alongside exposure history to evaluate potential health hazards and determine appropriate medical interventions.