Ninhydrin treatment as a screening method for the suitability of swabs taken from contact stains for DNA analysis.

More and more swabs containing unknown traces of biological material are submitted for forensic DNA analysis. Most of the samples are swabs taken from handled items such as tools, weapons and handles etc. Therefore, we tried to develop a screening method in order to focus the investigation on samples containing biomolecules, such as amino acids which might be associated with nucleic acids. A total of 285 swabs taken from various items collected during crime scene investigations were treated with ninhydrin which leads to a purple colour for samples containing amino acids. Of the swabs 158 were classified as ninhydrin positive and 76% of these samples yielded DNA profiles that fulfil the criteria for inclusion in the German national DNA database (profile frequency greater than 1 in 100,000) or in DNA mixtures which could at least be compared with suspects. In comparison only 9% of the 127 samples shown to be ninhydrin negative, revealed a usable DNA profile. Consequently, ninhydrin treatment was found to be an effective screening method which resulted in an increase in the rate of successfully typed samples and subsequently in a reduction of the costs due to the lower number of samples that needed to be typed.