Forensic Serology and Biological Fluid Analysis¶
Summary¶
This chapter introduces forensic serology — the scientific analysis of biological fluids as evidence. Students begin with the cellular composition of blood (erythrocytes, leukocytes, platelets, and plasma), then study the hierarchy of presumptive tests (Kastle-Meyer, Luminol, Fluorescein) and confirmatory tests used to establish the presence of blood. ABO and Rh blood typing are covered in depth, including the agglutination chemistry that underlies these reactions. Secretor status, and the detection of semen (PSA test), saliva (amylase), and urine round out the chapter. Students will understand why both presumptive and confirmatory testing are required before biological fluid evidence is admissible in court.
Learning Objectives¶
By the end of this chapter, investigators will be able to:
- Identify the four components of blood and explain the forensic significance of each.
- Distinguish between presumptive and confirmatory tests and explain why both are required before biological fluid evidence is admissible.
- Describe the ABO blood typing system and the agglutination chemistry that underlies it.
- Explain how secretor status affects the value of blood typing evidence.
- Select the appropriate detection test for semen, saliva, or urine.
Concepts Covered¶
This chapter covers the following 16 concepts from the learning graph:
- Blood Composition
- Erythrocytes
- Leukocytes
- Platelets and Plasma
- Presumptive Blood Tests
- Kastle-Meyer Color Test
- Luminol Test
- Fluorescein Test
- Confirmatory Blood Tests
- ABO Blood Typing
- Rh Factor
- Agglutination Chemistry
- Secretor Status
- Semen Detection (PSA Test)
- Salivary Amylase Test
- Urine Detection
Prerequisites¶
This chapter builds on concepts from:
Welcome, Investigators!
Biological fluids are among the most powerful forms of physical evidence — they carry cellular material, antigens, proteins, and DNA that can connect people to scenes, objects, and each other. But they can also fool you if you rush straight to conclusions without proper testing. This chapter teaches you to test first and conclude carefully. Follow the evidence — and verify every step of the way.
The Composition of Blood¶
Before any serological test can be interpreted, investigators need to understand what blood actually contains. Blood is a specialized connective tissue that transports oxygen, nutrients, hormones, and waste products throughout the body. It is composed of four main components:
Erythrocytes (red blood cells, or RBCs) make up approximately 45% of blood volume. These biconcave disk-shaped cells lack a nucleus in their mature form and are responsible for oxygen transport via hemoglobin. Erythrocytes carry the ABO and Rh antigens on their surface membranes — the basis of blood typing. When blood dries at a scene, erythrocytes contribute the red-brown color of dried blood.
Leukocytes (white blood cells, or WBCs) make up less than 1% of blood volume. There are several types (neutrophils, lymphocytes, monocytes, eosinophils, basophils), and they collectively form the cellular immune system. Forensically, leukocytes are the primary source of nuclear DNA in a blood sample — they contain nuclei, whereas mature erythrocytes do not.
Platelets (thrombocytes) are cell fragments (not complete cells) that circulate in the bloodstream and initiate the clotting cascade when a vessel is injured. They make up a small fraction of total blood volume. Forensically, platelets contribute to the clotting of blood that is deposited at a scene, influencing the ultimate appearance and distribution of stains.
Plasma is the liquid component of blood — a straw-colored aqueous solution of water (~91%), proteins (albumin, globulins, fibrinogen, clotting factors), hormones, nutrients, and electrolytes. Plasma contains no cells; it is the medium in which the cellular components are suspended.
After blood is deposited at a scene and clotting occurs, the fluid that separates from the clot is called serum — plasma minus the clotting proteins. Many serological tests detect proteins present in serum.
Presumptive and Confirmatory Testing: The Two-Step Rule¶
A critical principle underlies all forensic serological testing: presumptive tests come first; confirmatory tests establish identity. This hierarchy exists because most presumptive tests are not specific for blood — they can produce positive results for other substances. Courts require confirmatory testing before biological evidence is admitted.
Presumptive Blood Tests¶
A presumptive test is a rapid screening test that indicates the possible presence of blood. A positive result justifies further confirmatory testing; it does not prove blood is present. All three major presumptive tests work through the same chemical mechanism: they detect the peroxidase-like activity of hemoglobin, the iron-containing protein in erythrocytes. Hemoglobin catalyzes the oxidation of a chromogenic reagent (a color-producing compound) in the presence of hydrogen peroxide.
Kastle-Meyer Color Test (Phenolphthalein Test): A swab of the suspected stain is treated with phenolphthalein reagent and hydrogen peroxide. If hemoglobin is present, the phenolphthalein is oxidized to phenolphthalein acid, producing a bright pink-to-red color change. This is the most widely used field-portable presumptive blood test. Positive results can occur with vegetable peroxidases (certain plants) and some metal oxides, so confirmation is required.
Luminol Test: Luminol is a chemiluminescent reagent that produces a blue-white glow when it contacts hemoglobin in the dark. Unlike the Kastle-Meyer test, luminol does not require visible light — it can detect bloodstains that have been cleaned, diluted, or are invisible to the eye. The test is highly sensitive (detecting blood diluted 1:10 million) but also prone to false positives from bleach, copper, and certain plant materials. A second critical limitation: luminol is destructive to DNA — it should be used after DNA collection areas are identified.
Fluorescein Test: Fluorescein is a chemiluminescent reagent (like luminol) that requires UV/ALS (alternate light source) illumination to produce visible fluorescence rather than visible-light chemiluminescence. It is less destructive to DNA than luminol and is preferred in laboratories when DNA recovery is anticipated.
Common Mistake
A positive Luminol result does not mean blood is present — it means something with peroxidase activity is present. Luminol-positive results from bleach cleaners, copper pipes, and soil have led investigators astray. Never stop at a presumptive positive. Always confirm with a species-specific confirmatory test before reporting blood in your case notes.
Confirmatory Blood Tests¶
A confirmatory test specifically identifies the substance in question — in this case, blood — and often determines whether it is human in origin. Common confirmatory tests for blood include:
- Teichmann crystal test (hemin crystal test) — creates microscopic hemin crystals visible under a microscope when hematin is reacted with glacial acetic acid; the distinctive rhomboid crystal shape confirms blood
- Takayama crystal test (hemochromogen test) — creates feathery pyridine-hemochromogen crystals; also confirms blood microscopically
- Immunochromatographic strips (rapid immunoassay strips) — commercially available lateral flow strips that use antibodies specific to human hemoglobin to confirm both the presence of blood and that it is human in origin; results in minutes
The immunochromatographic strips are now the standard confirmatory tool in most forensic laboratories because they are rapid, require minimal sample, and simultaneously confirm human origin.
ABO Blood Typing¶
ABO blood typing is one of the oldest forensic identification systems. Developed by Karl Landsteiner in 1901 (for which he received the 1930 Nobel Prize in Physiology or Medicine), the ABO system classifies blood into four types based on the presence or absence of specific glycoprotein antigens on the surface of erythrocytes.
Before describing the four types, two terms need definition. An antigen is a molecule on a cell surface that the immune system can recognize. An antibody is a protein produced by the immune system that binds specifically to a particular antigen, often triggering agglutination (clumping).
The four ABO blood types are:
| Blood Type | Antigens on RBCs | Antibodies in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A antigen | Anti-B antibody | A, AB | A, O |
| B | B antigen | Anti-A antibody | B, AB | B, O |
| AB | A and B antigens | Neither anti-A nor anti-B | AB only | A, B, AB, O |
| O | Neither A nor B | Both anti-A and anti-B | A, B, AB, O | O only |
Agglutination Chemistry¶
Agglutination is the clumping of cells that occurs when antibodies bind to the antigens on their surface. In blood typing, it is the visible endpoint that indicates a reaction.
When anti-A antibody is added to a blood sample containing A antigens on the red blood cells, the antibody molecules bind to multiple A-antigen-bearing cells simultaneously, crosslinking them into visible clumps. If the blood sample has no A antigens, no agglutination occurs. By testing a blood sample against both anti-A and anti-B reagents (and a control), the ABO type can be determined.
Rh Factor¶
The Rh factor (or Rh antigen, specifically the D antigen) is a separate blood group system. A person is Rh positive if they have the D antigen on their erythrocytes; Rh negative if they do not. Approximately 85% of people are Rh positive.
For forensic purposes, the Rh factor adds an additional discrimination variable. When combined with ABO type, the full blood type description (e.g., "O positive," "AB negative") narrows the pool of possible contributors.
Diagram: ABO Blood Typing Interactive Simulator¶
ABO Blood Typing Interactive Simulator
Type: microsim
sim-id: abo-blood-typing
Library: p5.js
Status: Specified
Learning Objective: Explain the ABO blood typing procedure and interpret agglutination results to determine blood type (Bloom Level 2 — Understand; verb: interpret).
Bloom Level: Understand (L2) Bloom Verb: Interpret
Canvas layout: - Main view (~65%): two test wells side by side (one labeled Anti-A, one labeled Anti-B) - Control panel (~35%): blood type selection (hidden initially), reaction results, explanation panel
Visual elements: - Two circular test wells on a slide background - Animated agglutination: when a reaction occurs, small circles (RBCs) cluster together into visible clumps - When no reaction occurs, the circles remain evenly dispersed - A color indicator: agglutination (positive) shown as dark red clumped mass; no agglutination (negative) as smooth reddish tint
Interactive controls: - A "Select Blood Sample" dropdown with four mystery samples (each corresponds to a blood type) - "Add Anti-A" and "Add Anti-B" buttons that trigger the well animations - A "Determine Blood Type" button that reveals the correct answer after the student makes their prediction - Rh factor add-on: a third well with Anti-D reagent can be shown/hidden
Data Visibility Requirements: - Stage 1: Show two empty wells - Stage 2: Student adds reagents; animation shows agglutination or smooth suspension - Stage 3: Student predicts blood type; "Reveal" button shows correct type with explanation
Behavior: - Type A: Anti-A well agglutinates; Anti-B well stays clear - Type B: Anti-B agglutinates; Anti-A clear - Type AB: Both wells agglutinate - Type O: Neither well agglutinates
Instructional Rationale: The Understand objective (interpret agglutination results) requires the learner to observe the reaction and connect visual output to underlying chemistry — best achieved by animated simulation rather than text description.
Color scheme: Dark red for agglutination, light reddish tint for clear suspension, white slide background.
Secretor Status¶
Approximately 80% of the human population are secretors — people who secrete ABO blood group antigens not only on their red blood cells but also in other body fluids, including saliva, semen, vaginal secretions, sweat, and tears. The remaining 20% are non-secretors — their body fluids do not contain soluble ABO antigens.
Forensic significance: if a semen or saliva stain is recovered from a scene and the depositor is a secretor, the ABO blood type can be determined from that stain even if no blood is present. This was an important identification tool before DNA profiling became routine and can still provide supporting information when DNA evidence is degraded.
Non-secretor status is not rare (roughly 1 in 5 people), so the absence of detectable blood group antigens in a non-blood fluid sample does not exclude a suspect — it may simply mean they are a non-secretor.
Detection of Other Biological Fluids¶
Semen Detection: The PSA Test¶
Semen detection is relevant in sexual assault investigations. The most widely used semen identification test is the PSA (Prostate-Specific Antigen) test, also known as the p30 test. PSA is a protein produced almost exclusively by the prostate gland and is present at high concentrations in semen. The test uses antibody-based immunochromatographic strips (the same technology as confirmatory blood strips) that detect PSA specifically.
A positive PSA test confirms the presence of semen but does not identify who deposited it — that requires DNA profiling of the sperm cells or seminal fluid nucleated cells (spermatogenic cells) present in the stain.
If a male subject has had a vasectomy, semen is still present (and PSA still tests positive) but sperm cells are absent — the DNA must come from the nucleated cells in the seminal plasma.
Salivary Amylase Test¶
Saliva is identified by detecting salivary amylase (alpha-amylase), an enzyme present at very high concentrations in saliva and at much lower concentrations in other body fluids (sweat, blood). The amylase test uses a substrate (starch) that amylase degrades; the degradation is detected colorimetrically or by a starch-iodine color reaction.
Saliva evidence appears on envelopes, stamps, bite marks, cigarette butts, drinking vessels, and any surface a person licks or spits on. High amylase activity strongly suggests saliva; very high activity (100 times background) is essentially diagnostic of saliva.
Urine Detection¶
Urine is identified by detecting urea (using urease enzyme assay or chemical dipstick test), creatinine, or uric acid. Urine can be relevant in cases involving evidence of occupation of a space over time, staged scenes, or specific criminal circumstances.
Key Concepts Review¶
The following table summarizes the major concepts from this chapter:
| Concept | Definition |
|---|---|
| Erythrocytes | Red blood cells; carry ABO/Rh antigens; primary source of blood color |
| Leukocytes | White blood cells; primary source of nuclear DNA in blood samples |
| Platelets | Cell fragments that initiate clotting; affect stain morphology |
| Plasma | Liquid component of blood; contains proteins and clotting factors |
| Kastle-Meyer Test | Presumptive blood test; detects hemoglobin peroxidase; pink result is positive |
| Luminol | Presumptive test; chemiluminescent blue-white glow; high sensitivity; damages DNA |
| Confirmatory Test | Species-specific test (immunoassay strip) required before court admission |
| ABO System | Four blood types based on surface antigens; A, B, AB, O |
| Rh Factor | D antigen; positive (85%) or negative (15%); additional typing variable |
| Agglutination | Visible clumping of RBCs when antibody binds to antigen |
| Secretor Status | ~80% of people secrete ABO antigens in body fluids (saliva, semen) |
| PSA Test | Confirms semen via Prostate-Specific Antigen detection |
Challenge: Which Test First?
An investigator finds a dark stain on a wall at a suspected crime scene. The stain is dark reddish-brown and approximately 15 cm in diameter. The scene has been partially cleaned.
List the testing sequence you would follow, naming each test, its purpose, and why the sequence matters. Assume you also want to preserve DNA evidence.
Answer: (1) Photography — document the stain in situ before any testing. (2) Kastle-Meyer presumptive test on a small swab from the edge — if positive, justifies further testing. If negative, saves resources. Does not destroy DNA on the main stain. (3) Collect DNA swab from the main stain area — using a sterile cotton swab with sterile water, before any destructive tests. DNA is the highest-value evidence. (4) Immunochromatographic confirmatory strip on a second small swab — confirms human blood. Luminol should NOT be used here because it damages DNA and the stain is already visible.
Case Closed — For Now
You now understand the language of biological fluids — from the composition of blood to the two-step testing hierarchy to the ABO chemistry that has been used in forensic casework for over a century. Chapter 7 takes this further by examining what happens when blood is not just a fluid but a dynamic projectile: bloodstain pattern analysis. Follow the evidence!