How long does a fingerprint last?

Genetics in Forensic MedicineEphemeral fingerprints

A crime scene. The police find a body. When examining the victim, forensic doctors discover a bruise on the upper arm. Apparently the perpetrator detained the victim. Scientists from Freiburg had investigated in two EU studies which traces of the perpetrator can be detected on the skin.

"The investigation made it clear that papillary ridges can be visualized and that DNA could also be found from them." According to Sari Eble from the Institute of Forensic Medicine at the Freiburg University Medical Center, not only can the perpetrator's classic fingerprints be found on the victim's skin, but also a genetic fingerprint, as skin cells are automatically left behind with every touch.

The conclusion at the time: In 19 percent of the cases, these traces could have led to an identification of the tracer. In a follow-up study, the researchers at the University Medical Center in Freiburg have now investigated how long these genetic traces of the perpetrator can still be detected on the skin, even when putrefaction has already set in. "We did the whole thing on pigskin and then put fingerprints on so that we could just look at the changes in DNA over the course of days."

"Material can no longer be evaluated after three to four days"

The investigation on pig skin was necessary because such tests on human corpses could not be carried out for ethical reasons. Sari Eble asked her lab manager to leave his fingerprints on the pigskins. The question was: How many skin cells are there and how long can the researchers use them to create a genetic fingerprint?

"The sample storage was such that the pig skins were stored in a humid chamber within the incubation period and that also at room temperature, between 18 and 22 degrees." Over the next eight days, the pig skins gradually began to decompose. The scientists looked at how long the laboratory manager's DNA traces remained on pig skin, or how quickly they began to decompose in the course of putrefaction. Sari Eble regularly took small skin samples.

"Every 24 hours approximately, we also had shorter periods of time at the beginning to find out how quickly the DNA naturally degrades. However, it turned out that within three or four days the material can no longer be evaluated."

DNA lasts longer on smooth surfaces like glass

After 24 hours, 96 percent of the original amount of DNA - i.e. the skin cells of the laboratory manager - was still present, the next day the rate was only 40 percent. Apparently the rotting skin also influences the decomposition of the foreign DNA traces. "Yes, you can say that this putrefaction of the pig skin naturally also promoted the degradation. If you put fingerprints on a smooth surface, such as on glass or something, nothing happens to the DNA for a relatively long time."

In order to check how long the DNA actually remains on the pig skin, the forensic scientists applied blood evenly to the pig skins in parallel. Because it was initially not clear whether enough skin cells were left behind on the laboratory manager's fingerprints every time. So the researchers applied 200 blood samples to the pigskins and took samples for eight days.

The blood tests confirmed the results of the first part with the fingerprints. It is therefore quite possible for future investigations at a crime scene to take a usable genetic fingerprint of the possible perpetrator from the victim's skin, even if a few days have passed since the crime.