Vaginal Fluid and Faecal Material Identification

Vaginal fluid and faecal material are identified in sexual assault and other casework using glycogen-based tests, ferning, RSID-Vaginal Mucosa, and microscopic and chemical tests for faecal origin. This topic covers the specific assays, their limitations, and why correct identification matters for interpreting mixed biological stains.

Last updated: 19 Jun 2026

Vaginal fluid is identified in forensic serology using a complementary battery of tests: glycogen detection with Lugol's iodine or periodic acid-Schiff staining, the ferning crystallisation pattern of dried mucus, and the RSID-Vaginal Mucosa immunochromatographic assay targeting vaginal epithelial proteins. No single test is both sensitive and specific across all hormonal states, so identification requires concordance across methods. Faecal material is confirmed by Ehrlich's reagent for urobilinogen combined with microscopic recognition of plant cell walls, starch granules, and muscle fibres. Correct identification of both fluids is operationally critical because it determines how DNA extraction is structured and how resulting profiles are attributed to contributors.

Sexual assault examinations generate complex mixed biological stains, and the analyst's first task is to work out what is present before DNA profiling begins. Semen receives the most analytical attention, but vaginal fluid and faecal material each provide independent forensic information. Vaginal fluid carries the epithelial cells that yield a victim's DNA profile and can help establish that vaginal contact occurred. Faecal material appears in anal assault cases, cases involving contamination at scenes, and some drug-trafficking concealment scenarios.

Identifying vaginal fluid is harder than identifying semen. There is no enzyme as abundant as prostate-specific antigen, no cellular element as distinctive as a spermatozoon. Instead, the forensic serologist works with glycogen stored in vaginal epithelial cells, the crystalline ferning pattern of dried mucus, and the RSID-Vaginal Mucosa immunoassay. None of these is a single definitive test; together they build a case.

Faecal material is forensically important in a distinct set of case types. In cases of anal penetration, faecal contamination on a suspect's clothing or body can corroborate victim accounts. In drug cases, body-packing examinations involve faecal material as context. Identification uses urobilinogen detection, microscopy for food-derived particles, and sometimes bacterial culture, each contributing a different level of specificity.

By the end of this topic you will be able to:

Describe the principles and limitations of glycogen-based tests (Lugol's iodine, PAS with diastase control) for vaginal fluid identification.
Explain how the ferning pattern is produced, what conditions enhance or suppress it, and why it is classified as a presumptive rather than confirmatory test.
State what RSID-Vaginal Mucosa detects, why it is more reliable in post-menopausal donors than glycogen-based methods, and how it complements semen detection assays.
Identify the chemical and microscopic markers used to confirm faecal material and explain why a combination of Ehrlich's reagent and microscopy is required.
Describe how body-fluid identification results direct the choice of DNA extraction strategy, specifically the rationale for differential extraction when vaginal fluid and semen are both present.

Key terms

Glycogen: A polysaccharide energy store present in vaginal epithelial cells. It is released when cells are lysed and can be detected by periodic acid-Schiff staining or iodine reagent. Glycogen is also the primary carbon source for Lactobacillus in the vaginal microbiome.
Ferning: The arborisation pattern formed when cervical or vaginal mucus dries on a glass slide, produced by sodium chloride and glycoprotein crystallisation. Visible by light microscopy; most pronounced mid-cycle. A presumptive indicator of vaginal or cervical mucus.
RSID-Vaginal Mucosa: Rapid Stain Identification of Vaginal Mucosa. An immunochromatographic lateral-flow assay using antibodies targeting proteins specific to human vaginal epithelial cells, providing higher specificity than glycogen-based methods.
Urobilinogen: A colourless bile pigment metabolite produced by intestinal bacteria from bilirubin. It is excreted in faeces (as urobilin/stercobilin) and detected in forensic analysis by Ehrlich's reagent, providing a chemical indicator of faecal material.
Periodic acid-Schiff (PAS) reaction: A histochemical staining method that oxidises polysaccharides (including glycogen) to produce aldehydes, which are then detected by Schiff reagent as a magenta colour. Used to detect glycogen-rich vaginal epithelial cells in smear preparations.
Ehrlich's reagent: A solution of para-dimethylaminobenzaldehyde in acid. It reacts with urobilinogen and related pyrrole-containing compounds to produce a pink-red colour, used as a chemical test for faecal material in forensic screening.

Why vaginal fluid identification matters in sexual assault cases

In a sexual assault involving vaginal penetration, swabs from the victim's vagina and the suspect's body or clothing may contain both vaginal epithelial cells and sperm cells in the same deposit. Extracting DNA without recognising this mixture risks interpreting a combined profile. Differential extraction, which separates sperm cells from non-sperm cells using differential lysis, is the standard response, but it requires the analyst to recognise that both cell types are likely present.

On a suspect's clothing, vaginal fluid can also appear without semen if there was contact but no ejaculation, or if the suspect attempted to clean away semen. Vaginal fluid identification in that context supports a claim of sexual contact independently of whether sperm are present.

Glycogen detection: Lugol's iodine and PAS staining

Glycogen is chemically related to starch and reacts with iodine reagents (Lugol's solution) to produce a red-brown to brown colour, distinguishable from the blue-black produced by starch. In a forensic vaginal fluid screen, the stain extract or a smear from a swab is treated with Lugol's iodine and examined microscopically. Glycogen granules within vaginal epithelial cells stain red-brown, providing both a chemical positive and cell morphology information in one step.

The periodic acid-Schiff reaction is more sensitive and specific for glycogen than Lugol's iodine. Periodic acid oxidises glycogen's vicinal diol groups to aldehydes, and Schiff reagent then forms a magenta complex. Inclusion of a diastase digestion step as a negative control (diastase removes glycogen enzymatically; a PAS-positive result that becomes PAS-negative after diastase confirms glycogen rather than other polysaccharides) improves specificity further.

Vaginal fluid identification workflow.

Ferning and microscopic examination

The arborisation or ferning pattern appears when cervical and vaginal mucus dries on a smooth surface such as a glass slide. High-estrogen mucus in the days around ovulation contains a high sodium chloride concentration and forms prominent, well-branched fern patterns. After ovulation, progesterone changes mucus composition and the pattern becomes fragmented or absent. In forensic applications, a small volume of swab extract is spread on a slide, air-dried, and examined under low-power microscopy.

The forensic value of ferning is speed and simplicity. A positive result during a sexual assault examination immediately supports the presence of vaginal or cervical secretion. The limitation is that ferning can also be produced by amniotic fluid (the test was originally used in obstetrics to detect amniotic fluid leakage), and saliva with high mucus content can produce partial patterns. Ferning is therefore a presumptive tool within a wider testing strategy.

Ferning pattern from dried vaginal mucus.

RSID-Vaginal Mucosa: immunological confirmation

RSID-Vaginal Mucosa (manufactured by the same Independent Forensics platform as RSID-Saliva and RSID-Urine) uses antibodies against proteins expressed in human vaginal epithelial cells. The specific target antigen was not publicly disclosed by the manufacturer at the time of initial validation, a common proprietary restriction. Validation studies showed positive reactions with vaginal fluid from multiple donors across different cycle phases, including post-menopausal donors, and negative reactions with semen, saliva, blood, and urine.

Test	Principle	Specificity concern	Best use
Lugol's iodine	Colorimetric glycogen detection	Liver, muscle, bacteria also contain glycogen	Rapid screen with cell examination
PAS + diastase	Histochemical glycogen confirmation	Requires diastase control for specificity	Confirms glycogen in cells
Ferning	Crystallisation pattern of mucus proteins	Amniotic fluid also produces ferning	Rapid presumptive in SA examination
RSID-Vaginal Mucosa	Immunochromatographic protein assay	Cross-reactants not fully published	Most specific current confirmatory test

One practical consideration is that RSID-Vaginal Mucosa can give positive results with vaginal fluid deposited alongside other body fluids, which is the norm in sexual assault cases. Because the antibody targets vaginal epithelial proteins rather than semen markers, it is complementary to p30/PSA-based semen detection: a sample can be positive for both.

Faecal material: urobilinogen, microscopy, and context

Faecal material is identified by a combination of methods. Ehrlich's reagent reacts with urobilinogen, a reduced form of bilirubin produced by bacterial action in the large intestine and reabsorbed or excreted in faeces. A positive Ehrlich's reaction on an extract gives a cherry-red to pink colour. The test is not exclusive to faeces, since urobilinogen also appears in urine and some foods, but at a crime scene the combination of a positive Ehrlich's reaction with the visual and contextual features of the stain is usually sufficient for a presumptive call.

Microscopy adds significant power. Faecal material contains recognisable food-derived structures: plant cell walls (thick, polygonal, birefringent under polarised light), starch granules (large, oval, PAS-positive), and striated muscle fibres if meat was in the diet. These structures do not appear in other body fluids and provide a near-confirmatory microscopic identification when present in combination with a positive Ehrlich's test.

Ehrlich's reagent: Detects urobilinogen and urobilin; pink-red colour is presumptive for faecal origin.
Microscopy of plant cell walls: Cellulose walls from dietary fibre are resistant to digestion and appear as rectangular or polygonal structures; birefringent under crossed polars.
Starch granules: Partially digested starch granules retain their characteristic morphology and give a blue-black reaction with Lugol's iodine, distinct from glycogen's red-brown colour.
Muscle fibres: Cross-striated skeletal muscle fibres from meat are recognisable by their characteristic periodicity under phase contrast or H&E staining.

Interpreting mixed stains: vaginal fluid, semen, and faeces together

Sexual assault swabs frequently contain more than one body fluid. Vaginal cells, sperm cells, blood from injury, and occasionally faecal material from anal trauma can all appear on a single exhibit. The body-fluid identification step is not optional logistics; it directly controls how the DNA extraction is set up and how the resulting profiles are interpreted.

Screen for semen
Presumptive acid phosphatase test, followed by RSID-Semen or p30/PSA assay. If positive, sperm-cell microscopy documents presence and quantity of spermatozoa.
Screen for vaginal fluid
Ferning on a dry smear, glycogen staining, and RSID-Vaginal Mucosa. Documents vaginal contribution independent of semen finding.
Screen for blood or faeces if indicated
Leucomalachite green or Kastle-Meyer for blood; Ehrlich's and microscopy for faeces. Guides interpretation of mixed profiles from anal or genital injury.
Differential extraction for DNA
If semen and vaginal fluid are both present, differential lysis separates sperm cells (resistant to lysis without dithiothreitol) from epithelial cells, yielding two fractions that are profiled separately to attribute contributor identities.

An important point for report writing: vaginal fluid identification on a suspect's clothing does not alone prove vaginal intercourse. Finger contact, oral contact, or secondary transfer from shared surfaces or bedding can also deposit vaginal epithelial cells on clothing. The body-fluid result must be interpreted alongside the full case picture.

Worked example

Vaginal fluid on a suspect's underwear

Confirming vaginal fluid in the absence of semen, and explaining the absence of ferning.

A case of alleged non-ejaculatory sexual assault involves a complainant who reports vaginal penetration. The suspect's underwear is submitted for examination. The complainant is post-menopausal.

Semen screen. Acid phosphatase test is negative across the exhibit. RSID-Semen strip is negative. No spermatozoa found on microscopy. Conclusion: no semen identified.
Vaginal fluid screen. A stain area is identified visually on the crotch region. A dry smear is prepared and examined for ferning: no pattern is seen, consistent with post-menopausal mucus that has low estrogen-driven mucin. Glycogen testing via Lugol's iodine gives only a faint reaction, also consistent with reduced glycogen in post-menopausal epithelium.
RSID-Vaginal Mucosa. The same extract gives a clear positive test band. The analyst notes that the antibody target is not glycogen-dependent and retains reactivity with post-menopausal vaginal mucosa in published validation data.
DNA profiling. The stain extract yields a female DNA profile matching the complainant's reference sample.
Report. Human vaginal mucosa was identified by RSID-Vaginal Mucosa. Glycogen-based tests were negative-to-weak, consistent with post-menopausal hormone status. A female DNA profile consistent with the complainant was obtained. No semen was identified. The finding is consistent with, but does not prove, vaginal penetration.

The case illustrates why a test battery is necessary. Relying only on ferning or glycogen would have missed the vaginal fluid in a post-menopausal donor. RSID-Vaginal Mucosa's protein-antibody basis makes it independent of the hormonal variables that affect glycogen and mucin production.

Check your understanding

Question 1 of 4· 0 answered

Why might glycogen testing give a weak or negative result for vaginal fluid in a post-menopausal donor?

Key Takeaways

Vaginal fluid is identified using a complementary battery: glycogen detection (Lugol's/PAS), ferning microscopy, and RSID-Vaginal Mucosa immunoassay, because no single test is both sensitive and specific across all hormonal states.
RSID-Vaginal Mucosa targets protein markers rather than glycogen, making it more reliable in post-menopausal donors where estrogen-dependent glycogen is reduced.
Ferning provides a rapid presumptive screen but is also produced by amniotic fluid and can be absent in low-estrogen states; it is one piece of evidence, not a standalone identifier.
Faecal material is identified by Ehrlich's reagent (urobilinogen), microscopic recognition of plant cell walls, starch granules, and muscle fibres, and contextual assessment; no validated immunoassay specific to human faeces is in routine use.
In sexual assault cases, body-fluid identification directs DNA strategy: knowing both vaginal fluid and semen are present triggers differential extraction to separate contributor profiles.

How is vaginal fluid identified in forensic serology?

Vaginal fluid identification relies on detecting glycogen, which is stored in vaginal epithelial cells and released when cells are lysed. The periodic acid-Schiff (PAS) reaction and iodine staining detect glycogen. The RSID-Vaginal Mucosa assay targets human epithelial cell-specific proteins using an immunochromatographic strip. A ferning pattern observed in dried vaginal discharge under a microscope provides additional presumptive evidence.

What is the ferning test for vaginal fluid?

When cervical or vaginal mucus dries on a glass slide, its sodium chloride and glycoprotein content crystallises into a fern-leaf pattern visible under a light microscope. The pattern is most pronounced at mid-cycle when estrogen levels are high. Ferning provides a rapid presumptive screen in sexual assault examinations, though it is not specific to vaginal fluid alone.

What does RSID-Vaginal Mucosa detect?

RSID-Vaginal Mucosa is an immunochromatographic lateral-flow assay that detects human vaginal mucosa using antibodies against proteins expressed in vaginal epithelial cells. Studies have found it positive with vaginal fluid and negative with blood, semen, saliva, and urine at concentrations relevant to casework. It provides higher specificity than glycogen-based tests for confirming vaginal fluid origin.

How is faecal material identified in forensic analysis?

Faecal identification uses a combination of approaches: urobilinogen (a bile pigment metabolite) detection using Ehrlich's reagent, visual inspection for characteristic colour and odour, and microscopy for undigested plant cell walls, starch granules, and muscle fibres. The combination is usually sufficient to confirm faecal origin, though no single specific immunoassay analogous to RSID tests is in standard forensic use.

Why does the distinction between vaginal fluid and semen matter in sexual assault casework?

In sexual assault cases, both vaginal fluid and semen may be present on the same exhibit. Identifying which biological material is which helps direct DNA analysis: vaginal cells carry the victim's profile, while sperm cells carry the depositor's. Misidentifying vaginal fluid as semen or vice versa could lead to incorrect assumptions about whether sexual contact involving ejaculation occurred. Ferning and RSID-Vaginal Mucosa help the analyst account for each fluid separately.

Test yourself on Forensic Serology with free, timed mocks.

Practice Forensic Serology questions

Found this useful? Pass it along.

Spotted an error in this page? Report a correction or read our editorial standards.

Key Takeaways

Your journey to becoming a forensic professional starts here.