Discrimination of wet or dried arterial and venous blood for forensic applications via eosin fluorescence lifetime

The arteriovenous oxygen content difference disappears after blood is exposed to air. The blood oxygen level-dependent imaging method is unable to differentiate oxygen-saturated arteriovenous bloods, and no other method exists for determining them. To overcome this limitation, we first proposed that the fluorescence lifetime of eosin added to wet whole blood (WB) can distinguish air-exposed wet arterial blood (AB) from venous blood (VB). We then demonstrated that the urea level in wet WB can be employed as an indicator for further confirmation of wet arteriovenous blood based on the change in the eosin lifetime in the urease-urea reaction. Because arterial clots mostly comprise aggregated platelets in dried blood whereas venous clots contain higher amounts of fibrin. The eosin-stained platelets and fibrin have different eosin lifetimes, and this difference between platelets and fibrin during clotting when the blood dries explains why the eosin lifetime can be used to differentiate air-exposed wet and dried blood. Moreover, we found that the eosin short lifetime (τ1), mean lifetime (τm), and lifetime-based scatter parameters can be applied to discriminate dried AB and VB clots. Indeed, fluorescence lifetime imaging microscopy (FLIM) could be a potential method to support bloodstain pattern analysis in criminal cases.