Hair as a Source of DNA and Toxicological Information

The follicle produces a keratin-rich shaft containing cuticle, cortex and sometimes medulla. Matrix cells near the dermal papilla proliferate during active growth; the papilla is not itself the population that becomes the shaft. Differentiation and keratinisation alter nuclei and organelles as material moves upward.

Anagen, catagen and telogen describe growth, regression and rest, while exogen describes shedding. Root morphology and adhering tissue help assess the specimen but do not create an absolute rule that only anagen hairs contain nuclear material or that every rootless fragment was naturally shed.

Nucleated root tissue can improve prospects for ordinary STR testing. A rootless shaft may retain highly fragmented nuclear DNA suitable for specialist methods as well as mitochondrial DNA. It should not be described as containing intact functioning mitochondria in every cortical cell.

The body site, condition, length, treatments and recovery circumstances should be recorded. These influence genetic and toxicological work differently. Scalp hair is commonly used for segmental toxicology; another body site must not simply be assigned the same growth rate or assumed to extend the timeline reliably.

A genetic examination may involve lysis, protein digestion, purification, quantification and typing. Method selection depends on the actual specimen and intended targets. Some root samples yield useful conventional STR results; others are limited by low quantity, damage, contamination or inhibitors.

A visible root is not a guaranteed full profile, and a published yield range is not a casework sensitivity threshold. Follow the laboratory’s validated input and interpretation procedures rather than automatically applying enhanced cycles. A mixed result can reflect adhering foreign material or other histories and needs assessment.

Plan microscopy, DNA and chemical examinations before washing, mounting, cutting or digesting a limited exhibit. A treatment useful for one assay can remove information needed for another. Document the portions used, controls and any material retained.

National database specifications, such as the US core STR set, are not universal assay definitions. Donor statistics depend on the actual results and assumptions; the number of loci alone cannot establish individual identification.

Mitochondrial testing can be useful when appropriate nuclear testing is uninformative. Depending on the method, it can examine defined control-region ranges or the whole mitochondrial genome. A sequence is described relative to an appropriate reference and reporting convention, with the analysed range and quality stated.

Higher starting copy number can help recovery in some hair specimens, but does not guarantee that mitochondrial DNA survives every exposure better than nuclear DNA. Method target length and sequence preservation also matter. A mitochondrial result cannot be predicted from microscopy alone.

Maternal relatives may share a sequence, and unrelated people may share it too. An agreement therefore does not establish a particular maternal relationship or unique donor. Heteroplasmy and mutation can produce variation among tissues or relatives, affecting the assessment of differences.

Appropriate population data and method guidance support interpretation. A simple database frequency is not the probability that the named person is the donor. A teaching example should not invent a numerical frequency for an unspecified sequence, and reports should distinguish genetic association from the hair’s transfer history.

Drugs and metabolites can enter growing hair through processes associated with the follicular blood supply. Sweat, sebum and external material can also contribute to findings. The relative contribution depends on the compound, environment and specimen, so the bloodstream is not a universal sole route.

Some basic compounds bind to melanin and other hair components. Pigmentation can influence concentration for particular analytes, but the direction and magnitude cannot be generalised to every drug or person. A concentration difference between two people does not establish a dose difference or identify pigmentation as the sole cause.

External contamination may arise from powder, smoke, contaminated surfaces or biological fluids. Surface material can penetrate or bind to hair and may not be removed completely by washing. Internal detection after washing is therefore not absolute proof of ingestion.

Metabolite patterns can strengthen interpretation when assessed with concentrations, ratios, assay capability and relevant contamination research. Benzoylecgonine must not be described as occurring only through a person’s metabolism: it can occur in seized cocaine and arise through chemical processes. Its presence alone does not resolve every contamination question.

Elemental analysis can measure metals or metalloids, but external deposition, species of the element, environmental background and preparation affect interpretation. Hair concentration is not a simple proportional measure of blood concentration or proof of poisoning. A historical hair analysis should not be presented as a settled verdict about a death.

Segmental analysis divides an oriented hair sample into measured portions and examines each separately. Scalp hair is often approximated at one centimetre of growth per month for interpretation, but variation between follicles and people, resting hairs and the interval before emergence blur timing.

A proximal three-centimetre sample can provide an approximate retrospective window under the stated assumptions. It does not correspond to three exact calendar months for every hair or resolve use on a specific day. A rootless fragment without secure orientation or growth context is especially unsuitable for a precise calendar reconstruction.

Sweat or sebum contributions, external contamination, treatment, weathering, alignment and analytical sensitivity can alter a concentration pattern. A positive-to-negative boundary is not a hard start or stop date. A negative segment does not prove abstinence throughout its supposed period, and concentrations do not reliably quantify consumption.

Body hair has different growth and resting characteristics, so segmenting it into one-centimetre portions does not simply create a longer precise timeline. Very long scalp hair can be analysed when appropriate, but distal material has its own weathering and representativeness limits.

Hair is generally unsuited to establishing immediate impairment or replacing acute blood or urine testing. In a suspected single exposure or drug-facilitated event, collection timing, analyte sensitivity and specialist protocols matter. The absence of a detectable hair finding does not exclude an exposure.

For a planned toxicology collection, obtain suitable authority or consent, document body site and treatment history, and collect the quantity required by the receiving laboratory. Scalp hair is commonly cut close to the scalp at the posterior vertex, with proximal orientation preserved. Pulling hairs is not the routine requirement for this purpose.

Keep the bundle aligned and package to preserve identity, dryness and orientation. Record collection date, site, length, approximate amount, colour, cosmetic treatments and relevant history. Questioned hairs and known-reference collections have different purposes and may require different sampling plans.

Decontamination and extraction must be validated for the analytes. No fixed sequence of methanol, dichloromethane and water is best for every drug. Harsh conditions can alter compounds or create misleading products; an alkali digestion is not automatically appropriate for all opioid metabolites.

Chromatographic mass-spectrometric methods such as LC–MS/MS or GC–MS can provide sensitive, selective measurements when validated. Assess identity criteria, calibration, recovery, matrix effects, limits, controls and measurement uncertainty. Cutoffs are decision thresholds for a specified application, not universal physiological boundaries.

Analyse wash results where the method calls for it, but do not use a simple wash-versus-digest comparison as an infallible ingestion test. Combine analyte-specific evidence with treatment history, metabolites, contamination possibilities and the case question. Clinical, workplace and legal requirements differ by jurisdiction and intended use.