Chapter 14
IN THIS CHAPTER
Having a look at what blood is
Discovering forensic uses for blood
Testing for paternity using blood types
Reviewing forensic uses for other bodily fluids
Your identity shows up in more than your driver’s license. Blood, sweat, and tears are just a few of the bodily fluids that investigators can use to determine a suspect’s or victim’s identity through a science called serology. These biological fluids are frequently found at the scenes of crimes, particularly violent crimes, and can serve not only to link a suspect to the scene but to identify an individual with a degree of certainty that matches or surpasses fingerprints. This degree of individuality is also useful in cases of questioned paternity and ancestry.
Blood is by far the most common bodily fluid left at a crime scene, and it’s the most useful, because it opens up many avenues of investigation for the forensics team. Some of these avenues depend upon how blood stains and spatters (see Chapter 6), and others depend upon blood’s chemical components and biological behaviors.
Serologists conduct chemical and biological tests on blood samples retrieved from crime scenes with an ultimate goal of determining who the blood came from. That kind of information can tightly link a suspect to the scene or indicate that the suspect was far away when the crime occurred. It can determine whether the blood on a suspect’s sock belongs to the victim or whether the blood found beneath the victim’s fingernails is that of the suspect or someone else. The coroner uses the science of serology to make these determinations.
Whenever whole blood is permitted to clot, and the clotted material is removed, the remaining yellowish liquid is called serum. Serum contains most of the proteins and enzymes of plasma, but none of the cells or clotting factors that were consumed during the clotting process.
From a forensic serologist’s point of view, the two most important components of blood are the RBCs and the serum. From these, serologists can determine the blood type of any blood samples or bloodstains.
The RBCs contain extremely important molecules called antigens, which not only instigate immune reactions within the body but also determine blood type. Antigens are designated as either A or B. People with Type A blood have A antigens on their RBCs, and those with Type B blood have B antigens. People with Type AB have A and B antigens, and RBCs of people with Type O blood have neither antigen.
Another antigen in the blood is called the Rhesus or Rh factor, which sometimes is referred to as the D antigen. If your RBCs have these antigens, your blood is deemed Rh positive, and if they don’t, Rh negative. So those with A-positive blood possess the A antigen and the Rh (D) antigen on their RBCs. People who have O-negative blood have neither the A or B, nor the Rh antigens on their RBCs.
Forget about identifying the victim or suspect for the moment; identifying a bloodstain is plenty challenging. Depending upon its age and the conditions to which it has been exposed, a bloodstain may look more like a purple paint splotch or a brown grease stain. Even an experienced eye can have a hard time differentiating between blood and, say, dried ketchup.
Whenever the ME and the serologist confront a liquid sample or a stain that might be blood, they work to answer these three questions:
Blood must be found in sufficient amounts and in good-enough condition for it to be useful in testing. Although modern techniques make testing even minute amounts of blood possible, if a sample is severely degraded, it may prove useless. Many chemicals can damage blood to the point that typing and DNA profiling can’t be performed. Putrefaction, or decay caused by bacterial activity within the cells, is another factor that can degrade a sample beyond repair. Because warmth and moisture promote bacterial growth, putrefaction proceeds much more rapidly under warm and wet conditions.
Most crime-scene samples that reach the serologist are dried and degraded at least to some degree. Dried samples have advantages and disadvantages. Although liquid blood typically offers more useful DNA samples, it’s more likely to be degraded or decayed because bacteria that putrefy the blood and tissues thrive in moist environments. Dried samples, on the other hand, lessen bacterial growth and are less likely to degrade.
To determine whether a given sample actually is blood (or some other substance), the serologist conducts tests of two basic types: presumptive and confirmatory. Presumptive tests typically are cheaper and faster. When they are positive, presumptive tests indicate a likelihood that blood is present but don’t establish that as fact. Confirmatory testing then is needed to be certain. When presumptive tests are negative, blood is not present, and the more expensive and time-consuming confirmatory tests can be avoided.
Presumptive tests for blood fall into two broad categories: those that yield a color reaction and those that cause a fluorescent (glowing) reaction.
Kastle-Meyer Color Test: Phenolphthalein is the active compound in the Kastle-Meyer Color Test. When blood, phenolphthalein, and hydrogen peroxide mix, hemoglobin (the oxygen-carrying molecule in red blood cells) causes the peroxide to react with the phenolphthalein, producing a dark pink color.
The major advantage of the Kastle-Meyer test is that the reaction is quick; the color change appears within a minute or two. The major disadvantage, however, is that certain vegetable products, such as potatoes and horseradish, also can cause the reaction to occur. Of course, potatoes and horseradish are not typically found at scenes where blood has been shed.
Other tests rely on reactions that cause blood to fluoresce, or glow, under ultraviolet light, revealing blood that can’t be seen with the naked eye. Perpetrators often attempt to scrub walls and floors clean, erroneously assuming that if blood can’t be seen, it can’t be found. Fortunately, that isn’t true.
At the crime scene, blood can be revealed by spraying a fluorescent chemical over the area thought to contain blood. The lights are lowered, and an ultraviolet light source is directed over the area, causing the bloodstains to literally glow in the dark. Not only is the likely presence of blood established, but the area of its distribution also is clearly defined. Spurts, spatters, drag marks, and foot- and handprints jump into view.
When blood is found at a crime scene, the ME then must determine whether the blood is indeed from a human and not from a dog or cat — or a duck-billed platypus, for that matter. Only after this determination is made can further testing be carried out to discover whose blood it is.
If a solution that contains an antiserum is brought into contact with a blood sample that contains the antigen it was designed to react against, a reaction occurs. This reaction produces an antigen-antibody complex that precipitates or falls out of solution and results in a visible line of precipitation settling out where the two solutions come into contact.
In other words, if an antiserum to human blood comes into contact with a solution that contains human blood, the reaction forms a visible line of precipitant or solid material between the two solutions. If, on the other hand, the blood is not human, no reaction occurs, and no line is created.
The modern crime lab contains antiserums to a variety of common animal bloods. Dog, cat, deer, cow, and sheep antiserums usually are readily available. Using these antiserums, a serologist can determine the species that shed the blood, a factor that can be important evidence in and of itself.
After determining that a blood sample is human, the serologist sets about determining its type. Standard blood typing uses liquid blood, and a positive reaction is indicated by agglutination, or clumping, of the RBCs. Agglutination can occur only if the blood is liquid and if the RBCs are intact.
However, crime-scene blood rarely is liquid; it’s more likely to be a stain — that is, clotted and dried — and the RBCs therefore have fragmented. Because the RBCs are not intact, they can’t agglutinate, and thus no antigen-antiserum typing reaction can be verified. The serologist cleverly gets around this problem with a technique that draws out the remaining antigens.
The bloodstained material is treated with blood antiserums.
The antibodies in the antiserums combine with the antigens.
The material is washed.
This step removes any excess antiserum-containing antibodies.
The sample undergoes elution.
Elution is a process that breaks down antigen-antibody bonds by heating them, thus freeing the antigen and the antibodies from one another. The antibodies that were bound to the antigens are washed off.
The eluted antibodies then are tested against known blood antigens, and their reactions are observed.
The antigens the antibodies react with reveal which antigens are present in the original unknown sample.
By simply typing the blood at a crime scene, investigators narrow their suspect list and completely exonerate some suspects by using the population distribution information for the four ABO blood types in Table 14-1.
TABLE 14-1 Population Distribution of ABO Blood Types
Blood Type |
Population Percentage with Blood Type |
O |
43% |
A |
42% |
B |
12% |
AB |
3% |
Besides determining the ABO type, serologists are able to further individualize blood samples. RBCs contain more proteins, enzymes, and antigens than those used in the ABO classification system. These include antigens with such catchy names as Duffy, Kell, and Kidd and intracellular enzymes such as adenylate kinase, erythrocyte acid phosphatase, and the very useful phosphoglucomutase (PGM).
PGM is an enzyme that appears in many different forms, or isoenzymes, and at least ten of them are fairly common. Regardless of ABO type, a particular individual can have any combination of the isoenzymes of PGM. The ME and the serologist use that fact to further narrow the list of suspects for further DNA analyses and confirmation that they were capable of leaving a particular bloodstain.
If the police find blood at the scene that matches the blood of a suspect who has Type AB, PGM 2 blood, the probability that that suspect is not the perpetrator is 2 in 1,000. Although not perfect, those odds still are much better than a coin toss. DNA testing, which I discuss in Chapter 15, then is used to further individualize the sample.
You inherit your blood type from your parents. For that reason, a serologist can assess paternity in many cases. The crime lab is often involved in paternity testing because paternity may be a critical component in determining child support, custody, and visitation. It also may play an important role in crimes and civil proceedings that involve kidnappings, insurance fraud, and inheritance conflicts.
ABO blood types, or phenotypes, come in only four varieties: A, B, AB, and O. But, for some blood types two genotypes, or gene pairings, are possible. A phenotype is what something looks like (in this case the ABO blood type), while the genotype is the underlying genetic pattern. We receive our ABO genes from our parents, one from Dad and one from Mom.
TABLE 14-2 Determining Possible Genotypes from Phenotypes
Phenotype |
Possible Genotypes |
A |
AA or AO |
B |
BB or BO |
AB |
AB |
O |
OO |
People with Type O blood must have an OO genotype. They can have neither an A nor a B gene because having one or the other dominates the O gene and produces either Type A or Type B blood.
A person with Type A blood can either receive an A gene from each parent and thus have an AA genotype or an A gene from one parent and an O gene from the other for an AO genotype. Remember, A is dominant, so when it is paired with the recessive O gene, the A gene determines blood type. People with the AA and AO genotypes both have Type A blood, but genetically speaking, they’re different.
In each of the scenarios presented in Figure 14-1, the child’s blood type is Type A, except when both parents donate an O gene. In the latter case, the child’s genotype and blood type (phenotype) respectively are OO and Type O. These parents can’t have any offspring who have Type B phenotype or BB, BO, or AB genotypes, because neither parent has a B gene to donate.
Blood typing can exclude paternity but cannot absolutely verify it. For example, a man with Type AB blood can’t father a child with Type O blood. So if a child has Type O blood, all men with the Type AB are ruled out as the child’s father. A man with Type A (genotypes AA or AO) blood can be the father, but only if he has an AO genotype. Men who have AA genotypes also are excluded. Men with the AO genotype, however, can’t be ruled out at this point.
However, DNA matching (see Chapter 15), which offers 99 percent certainty, when properly administered, is the gold standard for assessing paternity.
Blood isn’t the only bodily fluid that can lead crime-scene analysts to a criminal. Semen, saliva, and, in rare cases, vaginal fluid may hold the key to solving a crime. During sexual assaults, semen and saliva commonly are transferred to the victim, the victim’s clothing, or nearby surfaces. Saliva can be obtained from food, a cigarette butt, or even from the stamp and envelope of a threatening letter.
The first step in locating these bodily fluids is an examination of the crime scene with either an ultraviolet or laser light source, which causes the fluids to fluoresce, or glow.
These materials must be carefully collected and preserved, because as moist, biological materials, they are susceptible to putrefaction from bacterial growth.
At the scene of a sexual assault, the search for semen includes the corpse (in cases of murder) or victim, underwear, condoms, bed sheets and mattresses, carpeting, and flooring. A living victim undergoes a rape exam by a physician. This exam typically takes place in a hospital emergency room, where standardized rape kits are used. Turn to Chapter 12 to find out how investigators handle rape cases.
Tests for semen are either presumptive or confirmatory. Presumptive testing generally is based on the fact that semen contains a very high level of the enzyme acid phosphatase. Confirmatory testing relies on the presence of spermatozoa or prostate-specific antigen (PSA).
Presumptive testing: Acid phosphatase enzymes (AP) are a class of proteins that are common in nature and are found in many animals and plants. Semen contains a high level of acid phosphatase, which is produced by the seminal vesicles. This type of AP is called seminal AP or SAP. When SAP is found in a crime-scene fluid sample or stain, it provides presumptive evidence that semen is indeed present. Unfortunately, certain fruit and vegetable juices such as watermelon and cauliflower, some fungi, contraceptive creams, and even vaginal fluid itself can give a false-positive AP test.
Other presumptive tests search for the presence of two other components of semen: spermine and choline. Each of these tests is positive whenever crystals form after the sample is exposed to certain chemicals.
Finding out when sexual intercourse or rape occurred often is critical in forensics and courtroom proceedings. An accurate determination can implicate or exonerate a suspect.
Semen is a fairly resilient substance. Laundering and dry cleaning of stained clothing removes all traces of PSA and SAP, but on rare occasions traces of spermatozoa can be seen through a microscope. If the material is protected from extremes of temperature, harsh chemicals, and other unfavorable environmental conditions, dried semen stains can remain identifiable and usable for DNA analysis for many years. Just ask Bill about Monica’s blue dress.
Thus, matching secretor antigens is similar to blood typing and can eliminate someone but cannot conclusively identify anyone specifically as the assailant, because it’s too crude. DNA testing must be employed to make a conclusive match.
Saliva is an important bodily fluid to the forensic examiner. It can be recovered from everything from stamps to food and bite marks. More importantly, it can reveal ABO antigens and thus blood types in secretors and sometimes can yield enough DNA for profiling.
Saliva begins the digestive breakdown of carbohydrates into simple sugars as you chew your food. Amylase is the enzyme that accomplishes this task. Like AP enzymes, amylase enzymes are found in many animals and plants.
Testing for saliva involves testing for the presence of alpha-amylase, the primary amylase found in saliva. No confirmatory tests exist for saliva, only presumptive ones.
Detecting vaginal fluid is difficult, but it may be important in nonejaculatory rapes and penetrations with foreign objects. Swabs can be taken from a suspect’s penis or from any suspected foreign object. Testing depends upon the finding of glycogen-containing epithelial cells. Epithelial cells line the vagina, and glycogen is a starch that is stored within the cells.
The problem is that not all vaginal epithelial cells contain glycogen. Cells from young girls who have not begun menstruating contain none. Similarly, cells from postmenopausal women rarely contain glycogen. The amount of glycogen found in these cells likewise varies with the stages of a woman’s menstrual cycle. As a result, in many cases, the test comes back negative even though vaginal fluids are present.