Menstrual Blood versus Peripheral Blood

Peripheral blood (venous, arterial, or capillary) clots on exposure to air through the coagulation cascade, forming a fibrin mesh that traps red cells. Menstrual blood does not. The endometrium produces prostaglandins that induce vasoconstriction and smooth muscle contraction during shedding, but it also releases high concentrations of tPA and other fibrinolytic agents that prevent stable clot formation in the uterine cavity, allowing the lining to be expelled as fluid rather than coagulated material.

This fundamental difference in haemostatic behaviour has been the basis for forensic differentiation attempts since the 1980s. Early methods used fibrin clot formation tests: expose the stain extract to conditions that promote clotting and see if a clot forms. Peripheral blood forms a firm clot; menstrual blood forms none or a fragile, rapidly lysed one. The limitation is that old stains have degraded proteins that affect coagulation regardless of source.

The MMP family of enzymes remodels the extracellular matrix. MMP-10, specifically, is involved in degrading fibronectin and laminin during endometrial breakdown. Research groups, including Bauer and colleagues, have demonstrated that MMP-10 concentrations in menstrual blood are many times higher than in peripheral blood from the same donors, and that an ELISA assay can detect this difference in stain extracts.

The challenge for forensic application is stability. MMP-10 is a protein and degrades over time in dried stains, with degradation accelerated by heat and UV exposure. Studies using fresh stains showed good discrimination, but performance on aged stains was more variable. A stain deposited weeks or months before examination may give an intermediate or negative result even from genuine menstrual blood.

D-dimer is generated when cross-linked fibrin is cleaved by plasmin. In peripheral blood that has not undergone significant fibrinolytic activity, D-dimer levels are low. In menstrual blood, where high tPA activity drives active fibrinolysis throughout the shedding period, D-dimer accumulates to much higher concentrations. Clinical D-dimer tests (used widely in medicine to exclude pulmonary embolism) have been adapted for application to forensic stain extracts.

Studies have shown D-dimer concentrations in menstrual stain extracts are consistently and significantly higher than in peripheral blood stains of the same age, at least for fresh to moderately aged stains. An immunoturbidimetric or ELISA D-dimer assay on a stain extract provides a quantitative result that can be compared with a threshold, rather than a simple positive-or-negative band.

The mRNA approach to body fluid identification asks which genes are being actively transcribed in the cells present in a stain. Endometrial cells shed during menstruation express transcripts of genes involved in the menstrual cycle: LEFTY2 (left-right determination factor 2, expressed in endometrium during the secretory phase), GLYCAM1 (endometrial surface glycoprotein), and MMP-10 mRNA, among others. These transcripts are detectable by RT-PCR in menstrual blood but not in peripheral blood.

The advantage of mRNA profiling is selectivity: a transcript that is specifically expressed in the endometrium provides a direct cellular origin marker without relying on non-specific enzymes or degradation products. The disadvantage is lability. RNA degrades more rapidly than DNA or protein, and mRNA profiling of stains older than a few days to weeks gives increasingly poor results.

The distinction between menstrual and peripheral blood is not an academic exercise. Three main case types turn on it.

• Sexual assault: Blood on a suspect's penis, hands, or clothing could come from the victim's injury during a non-consensual act, or from consensual sex during the victim's menstrual period. If the blood is menstrual, it does not directly evidence injury. The distinction informs whether injury is an issue in the case.
• Violence investigation: A suspect's clothing with blood from an alleged assault victim needs to be assessed against the possibility that the blood came from a different, non-injury source. If the victim was menstruating at the time, blood transfer during consensual contact is a legitimate alternative. A serological result that identifies menstrual rather than peripheral blood weakens the injury narrative.
• Scene interpretation: Blood found at a scene may be from an attack or from menstrual shedding at the scene location at a different time. Pattern and distribution of the staining provides some context, but body fluid characterisation helps.

Critically, the DNA in menstrual blood is the same DNA as peripheral blood from the same individual. A DNA match to the victim or suspect does not tell the analyst which type of blood it is. Body-fluid characterisation must precede or accompany DNA profiling to provide the full interpretive picture.

Several research groups have published proof-of-concept results for MMP-10, D-dimer, mRNA markers, and combined panels. The gap between a research publication and a validated forensic assay is significant. Full validation requires testing on a large number of donors across a range of hormonal states, ages, cycle phases, and health conditions, followed by assessment of performance on aged, contaminated, and environmentally stressed stains, and then accreditation under laboratory quality standards.

As of current practice, the field position is that a trained analyst can offer a provisional assessment using a combination of fibrinolytic activity tests and, where available, MMP-10 or D-dimer assays, but the result must be reported with clear qualification that the distinction is not yet supported by a fully validated commercial method. The interpretation carries less weight than, say, a positive RSID-Semen result, which has extensive validation data behind it.