Blood as Biological Evidence

Whole blood contains plasma and formed elements: erythrocytes, leukocytes and platelets. Plasma is the liquid fraction before clotting; serum is the liquid remaining after clotting and differs in clotting-related components. The familiar approximate 55-percent plasma and 45-percent red-cell figures vary with the person and physiological state.

A typical diploid leukocyte nucleus contains roughly twice the DNA of one haploid chromosome set. Do not describe 46 chromosomes as containing only the haploid three-billion-base-pair amount. Cell count, volume, preservation and extraction influence yield.

A stain’s diameter does not reliably give its original blood volume: substrate, dilution, spreading and deposition alter appearance. No coin-sized stain guarantees a full profile, and smaller traces may also yield useful results. Sampling and testing should be guided by the question and measured sample quality.

Autosomal STR analysis examines inherited variants in nuclear DNA. The result can support a donor comparison under stated propositions and an appropriate statistical model. The number of loci alone does not determine a universal random match probability or establish guilt.

Blood can also contain mitochondrial DNA from leukocytes and platelets, with quantities influenced by the specimen and processing. Mitochondrial testing may be useful when a suitable nuclear test is uninformative, but is not an automatic next step in every blood case. Maternal relatives and unrelated people may share the tested sequence.

Y-linked and X-linked markers can address particular mixture or relationship questions. A Y haplotype is not a unique male identifier. Routine autosomal, lineage and sex-associated results need their own inheritance and assay interpretations.

An authorised reference should represent the person for the question being asked. Haematopoietic stem-cell transplantation can cause blood to contain donor-derived genetic material; buccal samples can also show complex results. Relevant medical history and suitable alternative references may therefore matter. An unexpected result should be investigated rather than assumed to prove misidentification.

The US CODIS/NDIS marker requirements are one database specification, not a universal typing kit. Other national systems have their own requirements. A donor comparison need not involve a database, and database eligibility is separate from whether a laboratory can obtain a profile.

A garment can carry blood, saliva, semen, contact traces, hairs and background material. The source of a DNA component is not established simply because it was sampled near a visible bloodstain. Multiple materials or deposition events can contribute to one sampled area.

Semen can contain sperm and non-sperm cells; saliva may contain epithelial cells, leukocytes and extracellular DNA. Contact traces need not come only from intact skin cells or direct contact by the detected donor. These differences influence screening, sampling and mixture expectations.

A hair with adhering tissue may suit conventional nuclear testing; a rootless shaft can contain fragmented nuclear DNA as well as mitochondrial DNA. Microscopy cannot identify one person or reliably assign a person’s racial category. Bone, teeth and soft tissue each offer possible DNA sources depending on their condition.

The examination plan should coordinate biological, trace, fingerprint and pattern work. Some treatments consume or alter material. Record locations before sampling, use separate tools or appropriate cleaning, and retain suitable material where possible for justified further examination. One extraction strategy is not automatically appropriate for every stain on an item.

Appearance and alternate-light examination can help locate areas for testing, but colour and fluorescence are not definitive identifications. Chemical blood screens commonly exploit haem-associated oxidation chemistry. Kastle–Meyer produces a pink reaction under the appropriate conditions; luminol produces chemiluminescence. Interfering substances can also react.

Use validated application and observation rules. A delayed colour change outside the procedure’s interpretation window must not automatically be called positive. Substrate comparisons can help assess background but do not exclude every local interferent. Reagent controls and documentation are essential.

Further examination may include an appropriate human-haemoglobin immunoassay or other validated identification method. Product targets, cross-reactions, extraction conditions and reading times differ. Absence of a control line makes the illustrated lateral-flow result invalid; a negative result can also reflect low target, damage, interference or an applicable high-dose effect.

A positive blood-associated result and a compatible human DNA profile can jointly inform a case, while retaining their separate limitations. Human DNA typing does not itself confirm blood. Nor is there a universal requirement to complete a specific human-blood test before DNA work may begin; the laboratory selects and reports a justified workflow.

Bloodstain pattern analysis addresses the distribution, shape and physical mechanisms of staining through specialist methods. It does not infer an exact event or impact velocity from one stain size. Sampling should preserve necessary pattern documentation before material is removed.

Photograph and identify an item or stain before collection where feasible. Use protective measures, appropriate DNA-controlled consumables and separate packaging to limit contamination and transfer. Sterility alone does not certify that a device is free of amplifiable DNA.

EDTA is appropriate for many DNA-oriented blood collections, but tube colour, preservative and storage requirements must be verified for the intended examination. Toxicology and other analyses may need different specimens. Follow the receiving laboratory’s protocol, including restrictions on freezing liquid whole blood.

An air-dried swab or stained garment is generally packaged to remain dry. Liquid samples need leakproof containment, and some wet specimens require prompt cold transport or other specified arrangements. Paper is not a universal container for every biological specimen.

Document identifiers, seals, handlers, transfers, conditions and departures from procedure. Obtain references through the authority or consent applicable to the case; not every known person must automatically provide one. Collection powers, retention and expert duties vary by jurisdiction.

Heat, moisture, microbial activity and chemical exposure can damage DNA and other targets. Light can produce lesions that affect copying. Cleaning products may remove or damage material and can also interfere with screening reactions; bleach can cause reactive blood screens as well as genetic damage.

Quantification and suitable controls help assess recoverable human DNA, inhibition and relative short-/long-target recovery. These are not clocks for stain age. Shorter targets can sometimes improve recovery, but extra PCR cycles or software cannot recreate missing information.

Probabilistic genotyping evaluates findings under specified propositions and a validated model; it is not a laboratory extraction method or an automatic solution to degradation. Assess dropout, stutter, mixtures and analytical quality using the actual results and applicable method limits.

A partial profile compatible with a fuller profile is not thereby proved to have the same donor. Evaluate its information and alternative explanations without transferring the full profile’s statistic to it. One observed allele can be a true homozygote or reflect loss of signal; a partial profile is not automatically single-source.

Treated collection cards can support preservation and appropriate reference workflows, but their performance depends on the product, specimen and storage. Do not promise decades of recovery for every card or cite a withdrawn Indian bill as an enacted database law. Storage and database obligations must be checked against applicable current requirements.