Prediction of DNA concentration in fingermarks using autofluorescence properties

During criminal investigations trace DNA samples, including fingermarks, are submitted to laboratories for short tandem repeat (STR) analysis. For most common STR analysis systems a minimum amount of input DNA is required. Upon intake by the forensic laboratory the DNA concentration is estimated using quantitative polymerase chain reaction (qPCR) analysis after which most fingermarks are excluded. To tackle the problem of unnecessary processing in the lab, our study aimed to develop a method, which is able to predict the DNA content in fingermarks directly at the crime scene. Upon excitation with a UV Crime-lite, fingermark residues have autofluorescent properties. We hypothesize that the intensity of the autofluorescence signal of the fingermark content correlates to the DNA concentration in fingermarks. In this study, 164 fingermarks were examined on their autofluorescence intensity when excited at 365 nm, the number of nucleated cells, their DNA concentration and the completeness of the STR profiles. No significant correlation was observed between the DNA concentration in fingermarks and the autofluorescence signal, indicating that a high amount of autofluorescence, thus a high amount of biomaterial, does not necessarily guarantee a higher amount of DNA. In addition, the completeness of the STR profiles did not correlate to the autofluorescence signal of fingermarks. A moderate correlation was found between the predicted DNA quantity, based on the number of nucleated cells and the DNA quantity. In summary, the autofluorescence signal of fingermarks cannot directly be used as a guide to select fingermarks for DNA analysis directly at the crime scene. However, predicting the amount of DNA using a sensitive and specific DNA staining method can probably be used to estimate the DNA concentration in touch samples.