The Lattes Test and What It Actually Gets You

The Lattes test is one of those forensic methods that everyone in the field has heard of but very few people actually use correctly. Leone Lattes developed the technic of blood grouping from dried stains in 1910, and it remained the standard way to determine ABO blood types from crime scene evidence for decades. The principle is straightforward enough: you take a suspected bloodstain, elute the blood from the surface, and run it through agglutination reactions using anti-A and anti-B sera. If the red blood cells clump when exposed to anti-A, the blood is type A. Clumping with anti-B means type B. Both means AB. Neither means type O. The problem is that the method is far less useful than most people assume.

Leone Lattes Contribution To Forensic Science: Where It Falls Apart

I spent years dealing with degraded samples from old casework, and the Lattes test has some brutal limitations that beginners never learn until they waste hours on a failed run. The biggest issue is that dried blood loses antigens over time. Once the stain is older than a few years, or has been exposed to heat, moisture, or sunlight, the A and B antigens break down. You end up with what looks like a type O result, which is technically correct for the test but completely useless for exclusion purposes. I had a case where a 15-year-old stain tested as type O by Lattes, but modern DNA profiling later showed the donor was type A. The antigens had degraded enough to give a false negative, which would have potentially excluded a suspect if we had relied on the old method. Another thing nobody warns you about is that the test only gives you a group classification, not a unique identification. Blood type A accounts for roughly 36 percent of the population in most Western countries. Finding type A blood at a crime scene narrows the pool somewhat, but in a city of any size, that is still tens of thousands of people. The Lattes test cannot tell you whose blood it is. It can only tell you what group it belongs to, and even then, only if the sample is fresh enough for the antigens to survive. The procedural details matter more than most people realize. You need to cut a small piece of the stain, usually around two to three millimeters, and place it on a glass slide. Add a drop of saline to elute the blood, let it sit for about five minutes, then mix in the antisera. You want to avoid using too much stain material because excess hemoglobin can interfere with reading the agglutination. I once ran a test where the stain was so old and concentrated that the reaction was nearly impossible to read under the microscope. The workaround was to dilute the eluate significantly and run a control with known type A and type B blood alongside it. Without proper controls, you are just guessing at results that may not be reliable.

The real shift in forensic science moved away from Lattes precipitation and agglutination methods toward serological testing like the Ouchterlony double diffusion test for species origin, and eventually toward DNA profiling. The Lattes test had its moment, but once PCR-based methods became viable in the late 1980s and early 1990s, there was no going back. DNA can identify an individual from a single cell, whereas the Lattes test can barely tell you the broad category of blood you are dealing with, and often fails entirely on old or damaged evidence. If you are working with historical cases from before the 1990s and only have Lattes-type data available, treat the results as what they are: limited screening information. They can exclude a suspect if the blood types do not match, but a match does not prove anything beyond the fact that the suspect falls within the same general blood group. Modern labs rarely perform the Lattes test anymore, not because the technique is flawed in theory, but because it simply cannot compete with the discriminatory power of DNA analysis.