Urine and Sweat Identification

Urine evidence is more common in casework than laboratory textbooks suggest. Sexual assault examinations routinely include clothing from both victim and suspect, and urine staining on underwear or bedding needs to be identified and distinguished from semen or vaginal fluid before DNA analysis proceeds. Drug premises often contain urine deposits that can be typed and DNA-profiled, sometimes identifying individuals present at the scene.

• Sexual assault casework: Urine from a victim who lost bladder control during an assault, or from the offender; urine on bedding or clothing must be distinguished from semen to direct DNA analysis.
• Drug premises: Urine deposits identify individuals who occupied a location and may contain drug metabolites for toxicological analysis of the same stain.
• Document and mail examination: Threatening letters occasionally have urine deposited intentionally. Identification of the fluid is the first step.
• Touch DNA and sweat: Items handled by a suspect, from door handles to weapons to clothing, carry eccrine sweat deposits. The biological context is usually inferred rather than confirmed.

Sweat enters forensic analysis most commonly as the carrier fluid for touch DNA. When a person handles an object, eccrine secretion (sweat) deposits proteins and shed cells onto the surface. The sweat itself is rarely identified as a body fluid in a report; rather, its presumed presence is what allows the analyst to justify attempting DNA extraction from a handling area.

Creatinine is the preferred biochemical screening marker for urine because its urinary concentration is relatively consistent between individuals and is far higher than other body fluids. A colorimetric Jaffe reaction (creatinine reacting with picric acid in alkaline conditions to form an orange-red complex) was historically used; modern laboratories often use enzymatic creatinine kits adapted from clinical analysers.

Urea is also abundant in urine and can be measured with urease-based enzymatic assays. It is less specific than creatinine as a urine marker because it is also present in sweat at lower but detectable concentrations. The two markers together provide stronger presumptive evidence than either alone.

The Tamm-Horsfall protein, also called uromodulin, is a glycoprotein encoded by the UMOD gene and expressed only in the epithelial cells of the thick ascending limb of the loop of Henle and early distal convoluted tubule. It is the most abundant protein in normal human urine, at approximately 50 mg per litre, but is absent or present only in trace amounts in other body fluids under normal conditions.

RSID-Urine uses a lateral-flow strip with monoclonal antibodies against Tamm-Horsfall protein. Validation studies by the manufacturer and independent laboratories have found it to be negative with semen, saliva, blood, vaginal fluid, and sweat at concentrations relevant to casework. The test detects urine diluted to approximately 1:100 to 1:500 depending on the starting concentration, which covers most forensic dilutions on fabric or flooring.

Eccrine sweat is a watery fluid produced by sweat glands distributed across nearly the entire body surface. It contains water, electrolytes (predominantly sodium and chloride), small amounts of protein, and shed skin cells. The protein content is low compared with blood or semen, which is why no reliable colorimetric or enzyme assay exists for sweat analogous to those available for other body fluids.

Dermcidin is the most studied candidate marker. It is a peptide expressed constitutively in eccrine sweat glands and released into sweat at concentrations in the range of tens to hundreds of micrograms per millilitre. The DCD gene encodes a 110-amino-acid precursor; proteolytic processing in sweat yields the 47-amino-acid isoform DCD-1 and the 48-amino-acid isoform DCD-1L, both of which are antimicrobially active. It is not expressed at comparable levels in other body fluids. Researchers have detected dermcidin in touch-DNA samples and demonstrated immunochromatographic detection in laboratory conditions. However, no RSID-Sweat or equivalent validated commercial assay had entered routine forensic use as of recent practice.

• Current practice: Sweat identity is inferred from context (handling area, no other fluid identified) rather than confirmed by a body-fluid-specific test.
• DNA yield: The number of cells in sweat deposits varies considerably by individual shedder status and the type of activity. High-touch or prolonged-contact scenarios yield better DNA.
• Report language: Analysts should say 'a DNA profile consistent with X was obtained from the exhibit' rather than 'X's sweat was present', since sweat cannot be confirmed as the carrier.

Urine stains at drug scenes or on seized clothing have two analytical applications. The body-fluid identification step establishes that the deposit is urine. A subsequent toxicological screen on the same or a parallel extract can detect drug metabolites, because urine is the primary excretion route for many controlled substances and their breakdown products.

Immunoassay screening (enzyme multiplied immunoassay technique, or EMIT, and similar platforms) adapted for dried stain extracts can provide presumptive drug identification. Confirmation by liquid chromatography-mass spectrometry gives quantitative and compound-specific data. The major analytes recovered from urine stains include opioid metabolites, benzodiazepine glucuronides, cocaine metabolite benzoylecgonine, and cannabis metabolite THC-COOH.

Urine degrades. Creatinine and urea are small molecules that can leach out of a stain on a porous substrate over time, especially with humidity cycling. Tamm-Horsfall protein is a glycoprotein that denatures under heat and acid conditions. A stain on the floor of a vehicle exposed to summer temperatures may give negative results for all three markers even though it was once urine.

For sweat, the absence of a confirmatory test means a negative body-fluid screen across all other markers (blood, semen, saliva, urine, vaginal fluid) is the best that can be said. The analyst can note that the deposit is consistent with a contact-DNA deposit but cannot positively identify the biological carrier.

• Always state which tests were performed and what each was looking for.
• For a negative RSID-Urine, note that Tamm-Horsfall protein may have degraded and that urine is not excluded.
• For sweat, use language like 'no body-fluid of note identified; the exhibit may carry contact-deposited material' rather than claiming sweat is present or absent.
• Tie the biological result to downstream analyses: if drug metabolites were found in a deposit where urine is plausible but not confirmed, the totality of chemical findings still supports interpretation.