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Quiz: Forensic Serology and Biological Fluid Analysis

Test your understanding of blood composition, presumptive and confirmatory testing, ABO blood typing, and biological fluid detection with these questions.

1. Which component of blood is the primary source of nuclear DNA, and why do mature erythrocytes not provide DNA for forensic analysis?

  1. Platelets, because they are the smallest component and therefore most concentrated in dried blood stains
  2. Plasma, because it carries dissolved DNA fragments shed from all blood cells during circulation
  3. Leukocytes, because they are nucleated cells containing chromosomal DNA; mature erythrocytes lose their nuclei before entering circulation
  4. Erythrocytes, because hemoglobin carries genetic information encoded in its protein structure
Show Answer

The correct answer is C. Leukocytes (white blood cells) are the primary source of nuclear DNA in blood because they retain their nuclei, which contain chromosomal DNA. Mature erythrocytes (red blood cells) eject their nuclei before entering circulation as part of their differentiation process, so they carry no nuclear DNA. This is why forensic DNA analysis from bloodstains relies on the small leukocyte fraction rather than the far more abundant red blood cells.

Concept Tested: Leukocytes

2. The Kastle-Meyer color test is a presumptive test for blood. What chemical reaction produces its positive result?

  1. Phenolphthalein is oxidized by hemoglobin's peroxidase-like activity in the presence of hydrogen peroxide, producing a pink-to-red color
  2. Luminol reacts with iron in hemoglobin to produce a blue-white chemiluminescent glow in darkness
  3. Ninhydrin reacts with amino acids in blood proteins, producing a purple Ruhemann's purple compound
  4. Silver nitrate reacts with chloride ions in blood serum, forming silver chloride that darkens on light exposure
Show Answer

The correct answer is A. The Kastle-Meyer test works because hemoglobin has peroxidase-like activity — it catalyzes the oxidation of phenolphthalein by hydrogen peroxide, producing a bright pink-to-red color change. This is the same enzymatic mechanism exploited by luminol and fluorescein tests. Because vegetable peroxidases and some metal oxides can produce false positives, a positive Kastle-Meyer result is presumptive only — confirmatory testing is always required before reporting blood in case documentation.

Concept Tested: Kastle-Meyer Color Test

3. An investigator wants to search a large crime scene area for blood that has been cleaned and is invisible to the naked eye. Which presumptive test is most appropriate, and what is its key limitation?

  1. Kastle-Meyer; it can detect trace blood on all surfaces but requires bright ambient lighting to observe the color change
  2. Luminol; it can detect diluted or cleaned blood but produces false positives with bleach and some plant materials, and it damages DNA
  3. Fluorescein; it detects blood in complete darkness without any chemical exposure risk to the investigator
  4. Teichmann crystal test; it produces microscopic crystals visible even when blood has been diluted to trace amounts
Show Answer

The correct answer is B. Luminol is a chemiluminescent reagent that produces a blue-white glow when it contacts hemoglobin — even blood diluted 1:10,000,000 or cleaned from surfaces. Its key limitations are that it produces false positives from bleach, copper, and certain plant materials, and — critically — luminol is destructive to DNA. It should only be used after DNA collection areas have been identified, or fluorescein should be preferred when DNA recovery is anticipated from the same areas.

Concept Tested: Luminol Test

4. Why must a confirmatory test follow every positive presumptive blood test result in forensic casework?

  1. Confirmatory tests are faster, so they verify the presumptive result before valuable investigation time is spent on it
  2. Presumptive tests detect peroxidase-like activity, which can come from non-blood sources; confirmatory tests specifically identify blood (and human origin)
  3. Courts require two independent analysts to perform the same test, which counts as confirmation under Daubert
  4. Confirmatory tests determine how much blood is present, establishing whether the amount is forensically significant
Show Answer

The correct answer is B. Presumptive tests detect the peroxidase-like activity of hemoglobin — but this activity is not unique to blood. Vegetable peroxidases, bleach, copper compounds, and soil can all produce false positives. Confirmatory tests (such as immunochromatographic strips using antibodies specific to human hemoglobin) identify the substance specifically as human blood. Courts require confirmatory testing before biological evidence is admitted, because presumptive positives alone are not species-specific or substance-specific.

Concept Tested: Confirmatory Blood Tests

5. In ABO blood typing, Type O individuals have neither A nor B antigens on their red blood cells. What antibodies are present in their plasma, and what clinical significance does this have?

  1. Neither anti-A nor anti-B antibodies, making Type O the universal recipient who can receive blood from any type
  2. Both anti-A and anti-B antibodies, making Type O the universal donor for red blood cells but requiring matched plasma
  3. Only anti-A antibodies, because B antigens are suppressed in Type O individuals
  4. Anti-Rh antibodies only, because the Rh factor determines plasma antibody production in Type O blood
Show Answer

The correct answer is B. Type O individuals have both anti-A and anti-B antibodies in their plasma, because their immune system has not been exposed to either antigen on their own cells (they have neither) and produces antibodies against both foreign antigens. This means Type O red blood cells (carrying no A or B antigens) can be given to recipients of any ABO type without triggering an agglutination reaction — hence "universal donor" for red cells. However, the plasma from Type O must be matched because it carries antibodies that react with A and B antigens.

Concept Tested: ABO Blood Typing

6. Agglutination is the visible endpoint of a blood typing reaction. What molecular event causes the visible clumping of red blood cells?

  1. Antibody molecules bind to antigens on multiple red blood cells simultaneously, crosslinking them into visible clumps
  2. Red blood cells release fibrinogen when exposed to foreign antibodies, initiating the clotting cascade
  3. The pH change caused by antibody addition causes the cell membranes to rupture and release their contents
  4. Anti-A and anti-B reagents dissolve the glycoprotein surface of red blood cells, causing them to collapse and aggregate
Show Answer

The correct answer is A. Agglutination occurs when antibody molecules — which have two antigen-binding sites — bind to antigens on the surfaces of multiple red blood cells simultaneously, physically crosslinking the cells into visible clumps. For example, anti-A antibodies bind to A antigens on different Type A red blood cells, pulling them together into a clump that can be seen with the naked eye. The absence of agglutination means no antigen-antibody match occurred.

Concept Tested: Agglutination Chemistry

7. Approximately 80% of people are "secretors." What does secretor status mean, and how does it affect serological evidence?

  1. Secretors have more sweat glands and deposit higher quantities of biological trace material at crime scenes
  2. Secretors secrete ABO blood group antigens not only on red blood cells but also in other body fluids (saliva, semen), allowing blood typing from non-blood samples
  3. Secretors have higher concentrations of salivary amylase, making their saliva easier to detect with the amylase test
  4. Secretors shed more skin cells per hour, providing more DNA from touch surfaces than non-secretors
Show Answer

The correct answer is B. Secretor status refers to whether a person secretes ABO blood group antigens in body fluids other than blood — including saliva, semen, vaginal secretions, sweat, and tears. Approximately 80% of people are secretors. This means a semen or saliva stain from a secretor can be blood-typed even without a blood sample present, which was an important identification tool before DNA profiling became routine and can still provide supporting information when DNA is degraded.

Concept Tested: Secretor Status

8. The PSA (Prostate-Specific Antigen) test is used to confirm the presence of semen. If a male suspect has had a vasectomy, what effect does this have on PSA test results?

  1. The PSA test will be negative because a vasectomy eliminates all prostate secretions from semen
  2. The PSA test result is unaffected — PSA is produced by the prostate gland and is still present in semen after a vasectomy; sperm cells are absent, but seminal fluid (and PSA) remains
  3. The PSA test will show reduced sensitivity because the absence of sperm cells lowers the protein concentration in semen
  4. A vasectomy makes DNA analysis impossible because sperm cells are the only DNA source in semen
Show Answer

The correct answer is B. A vasectomy cuts the vas deferens, preventing sperm cells from entering the semen. However, the prostate gland — which produces PSA — is unaffected by a vasectomy, so PSA is still secreted into the semen. A PSA test will still be positive. For DNA analysis, the forensic examiner relies on nucleated cells in the seminal plasma (spermatogenic cells) rather than sperm cells, which are absent post-vasectomy. The DNA-containing cells are still present in semen after vasectomy.

Concept Tested: Semen Detection (PSA Test)

9. An investigator swabs a sealed envelope flap for possible saliva evidence. Which test is most appropriate, and what enzyme does it detect?

  1. Kastle-Meyer test, detecting hemoglobin peroxidase to confirm human biological origin
  2. Salivary amylase test, detecting alpha-amylase which is present at very high concentrations in saliva
  3. PSA immunoassay strip, detecting prostate-specific antigen which is present in all oral secretions
  4. Luminol test, detecting the chemiluminescent reaction produced by salivary proteins in darkness
Show Answer

The correct answer is B. 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. The test uses a starch substrate that amylase degrades, detected colorimetrically or by a starch-iodine color reaction. Saliva is commonly deposited on envelope flaps, stamps, bite marks, and cigarette butts. Very high amylase activity (100 times background levels) is essentially diagnostic for saliva.

Concept Tested: Salivary Amylase Test

10. An investigator at a crime scene finds a dark reddish-brown stain and wants to preserve the best possible DNA evidence while also confirming the stain is blood. Which testing sequence is most appropriate?

  1. Apply luminol first across the whole scene to map all blood, then collect DNA swabs from luminol-positive areas
  2. Photograph first, then collect DNA swabs before any chemical testing, then apply Kastle-Meyer to a small peripheral swab, then confirm with an immunochromatographic strip
  3. Perform confirmatory immunoassay first on the whole stain, then collect DNA after typing is complete
  4. Apply ninhydrin first since it is the most sensitive test for biological fluids, then swab for DNA from the developed print areas
Show Answer

The correct answer is B. The correct sequence prioritizes DNA preservation: (1) photograph the stain in place, (2) collect DNA swabs from the main stain area before any chemical testing, since DNA is the highest-value evidence, (3) apply the Kastle-Meyer presumptive test to a small peripheral swab to test for blood without disturbing the main stain, and (4) confirm with an immunochromatographic strip on a second small swab. Luminol should not be used when DNA recovery is planned because it damages DNA. Ninhydrin detects amino acids in fingerprints, not blood.

Concept Tested: Presumptive Blood Tests