Blood evidence is a powerful tool for the crime scene investigator. Whether testing for DNA or examining the bloodstain patterns to reconstruct the events of a crime blood is a powerful witness. This fact is not lost on the criminal. This knowledge is rooted in the old saying “caught red-handed” in which a criminal with blood on his hands was thought to be guilty. So criminals have learned to clean crime scenes and evidence and CSIs have learned ways to recover it. Without getting too deep in the forensic weeds; cleaning efforts usually result in either diluting the blood or masking it. Using a washing machine is an effective way of diluting bloodstained clothing. Criminals also have easy access to washing machines so it’s not too surprising that they may utilize them to wash away evidence.
Some of you may already be asking “why not just throw the clothing away?” It’s a god question but to understand it you have to understand a criminal and what they value. A t-shirt may get thrown away like garbage but if the item is their favorite jacket, sports jersey, athletic shoe, or ball cap then they may just roll the dice. One key thing to remember about all criminals. They will clean a crime scene to a point they do not see the evidence. That doesn’t mean the evidence is gone, it’s just beyond the abilities of the criminal to see it. So…will washing clothes destroy blood evidence? Sort of.
I won’t reveal the current state of DNA detection, suffice it to say that researchers are making breakthrough’s all the time. I’ve written before about the durability of DNA evidence and some of the current case studies and research might blow your mind. On the matter of dilution there are some amazing reagents like Luminol that may detect blood at one part per million. Several years ago I conducted a study to see if we could detect bloodstain patterns on washed clothing. I didn’t have high hopes but I thought it may be possible. The long of the short is that a number of cotton shirts were stained with various bloodstain patterns and then subjected to a series of alternating wash and dry cycles. I used washing detergent with bleach and dried the items in a hot-air clothes dryer. The long of the short is that I was able to detect blood on the clothing after five alternating cycles of washing and drying. At the time I used horse blood and DNA testing wasn’t as inexpensive as it is today so I didn’t address that issue. I just wanted to see if the bloodstain patterns could be detected.
You may want to keep this in mind as you’re developing your story. If your bad guy washes his/her clothing you may want o have your good guy find it. This would also work with victim’s clothing that has been exposed to rain, submersion, etc. If you develop a DNA profile all the better! I certainly won’t criticize you for it.
There are a number of other blood reagents like Luminol, Fluorescein, and Leucocrystal Violet that seem to be much more commonly referred to in literature and on television but Amido Black is another one you might consider using in your story. Amido Black is a presumptive blood reagent (can’t discriminate between human and non-human blood) that is very useful in developing diluted or latent blood impressions like shoe prints. This makes it an effective tool in the bloodstain pattern analysts arsenal. Amido Black is a water or methanol based dye stain that reacts with the proteins in blood turning them a dark blue-black color). The process begins by “fixing” the possible blood impressions with a methanol wash prior to the application of the reagent. Sometimes this methanol is mixed directly with the reagent at the time of application. The solution can then be sprayed or poured over the testing area. Some smaller items of evidence can be “dipped” in trays of the reagent as well. The reaction may not be as “flashy” as one of the luminescent reagents but it can be just as effective.
The sensitivity of the reagent is thought to be about 1:10,000 (parts per blood dilution) and has no detrimental effects on subsequent DNA testing although if too much reagent is “washed” over the evidence the sample could become too diluted for DNA testing. It can be used to detect any latent mark (such as tire print, shoe print, fingerprint, or tool mark) in blood at the crime scene. Once the reagent is applied and the blood impression is developed then the visible impression needs to be “fixed” with a 5% sulfosalicylic acid. The print can then be photographed.
I have successfully used this reagent many times at crime scenes and in the laboratory. It provides exceptional detail provided there is contrast with the background. That is the one potential problem with this reagent. Because it turns the blood a dark blue-black color, you can’t use it effectively on dark colored surfaces. That is something you’ll want to keep in mind if using this reagetn in your novel. The video below presents a homicide case in which Amido Black was successfully used to develop bloody shoe impressions from the suspect at the crime scene.
Luminol is a common chemiluminescent blood reagent used at crime scenes and in the laboratory. Chemiluminescence means that the blood will glow a blue-ish green color through oxidation as opposed to fluorescence (excitation) after being illuminated by ultra-violet light with an ALS. Luminol can detect trace amounts of blood as low as one part per million. There are a number of formulas used but most consist of the reagent Luminol, hydrogen peroxide (oxidant), and distilled water. It has been shown to be very effective in detecting cleaned up blood and older blood degraded through time. A few years ago a colleague of mine received permission to test the floor joists below the spot where Lizzy Borden’s father was murdered. After 112 years he was still able to clearly see the blood flow draining down through the floor boards into the cellar!
Luminol does have some limitations however. It is not specific to human blood (presumptive test). That means that it will react to the blood of other mammals the same as it will with humans. In fact, Luminol will react with substances other than blood such as iron, copper, certain vegetables, and cleaners like bleach. Criminalists call this reaction a “false-positive“. A well-trained criminalist can often tell the difference between a Luminol reaction with bleach (for example) versus one with blood. Bleach reacts with a glittering appearance in bright points of light throughout the reaction area. Blood will have a more subtle and uniform glow.
Another limitation is the duration and conditions of use. Chemiluminescence is typically only visible in near to total darkness. This can present a real challenge for criminalists, especially when working an outdoor crime scene. Most of the time you may have to do your examination and photography at night. Items like cars, clothing, weapons, etc can be brought into the interior spaces of the laboratory and processed there. If you must process a room in the crime scene you’ll have to block the windows with black plastic sheeting because shades and drapes simply won’t cut out enough light. Some scientists also believe that Luminol is a “potential carcinogen” and should be used with the appropriate respiratory protection.