Saliva Identification: Amylase and RSID-Saliva

Saliva is deposited during biting, oral contact, speaking at close range, spitting, and the handling of food or drink items. Each of these actions produces a biological trace that, once dried, is chemically stable. Salivary alpha-amylase has been detected in stains stored at room temperature for over a year, which means collection does not always have to be immediate, though delay adds uncertainty.

The forensic relevance goes beyond saliva identification itself. Saliva contains epithelial cells shed from the oral mucosa, and those cells carry nuclear DNA. Identifying a stain as saliva therefore sets up the next analytical step: STR profiling to establish who deposited it. The amylase test is not the endpoint; it is the gateway.

• Bite marks on skin: A swab over the bitten area can recover saliva deposited during the bite, linking a biter's DNA to a victim.
• Cigarette butts and cups: Contact with lips deposits both amylase and buccal cells, often in sufficient quantity for profiling.
• Envelopes and stamps: Licking gummed surfaces leaves a thin film of saliva; the double-swab technique recovers this even from old mail.
• Sexual assault cases: Oral sexual contact leaves saliva on skin, clothing, or bedding, where amylase testing helps interpret what mixed biological material is present.

Phadebas tablets were developed commercially as a clinical diagnostic for salivary amylase in the 1960s and were adopted by forensic laboratories as a screening tool shortly after. Each tablet contains an insoluble starch cross-linked to a blue dye. When alpha-amylase cleaves the starch backbone, the attached dye becomes soluble and releases into solution, producing a blue colour measurable by eye or by spectrophotometer at 620 nm.

1. Extract the stain
   Cut or swab the substrate and elute it in a small volume of distilled water. The extract volume must be consistent across a run so that dilution effects are comparable.
2. React with Phadebas
   Add a portion of Phadebas tablet (or a pre-dissolved working solution) to the extract and incubate at 37°C for 30 minutes. Enzyme activity proceeds in proportion to the amylase concentration in the sample.
3. Stop and measure
   Add sodium hydroxide solution to stop the reaction. The absorbance at 620 nm is proportional to amylase activity. Results are often expressed in units per ml and compared with a threshold for a positive call.
4. Interpret against controls
   A positive control (known saliva dilution) and a negative blank must run alongside samples. A positive result exceeding the threshold warrants further confirmatory testing.

RSID-Saliva is manufactured by Independent Forensics (now part of Thermo Fisher Scientific) and uses lateral-flow immunochromatography, the same platform as a home pregnancy test. The test strip carries two antibody zones. The first is loaded with monoclonal antibodies specific to human salivary amylase; the second is a control line. When extract is applied to the sample well, salivary amylase in the extract binds the detector antibody, migrates along the strip, and produces a visible pink or red line at the test zone.

Validation studies have found RSID-Saliva to be negative or only weakly reactive with semen, blood, vaginal fluid, and sweat at concentrations relevant to casework, which makes it genuinely useful as a confirmatory screen. That said, a small number of studies have reported that very high concentrations of pancreatic amylase can occasionally give a faint test band; context and the full case picture always matter.

The double-swab technique, described by Sweet and colleagues in 1997, remains the standard for bite-mark saliva recovery. A sterile cotton or foam swab is moistened with distilled water and rolled over the suspected deposition area in a circular pattern, then immediately followed by a dry swab to pick up the material loosened and rehydrated by the first pass. Both swabs are submitted.

• Timing: Collect as soon as possible. Amylase survives well on dried surfaces, but cellular DNA degrades faster, especially on skin exposed to sunlight or friction.
• Envelopes: Swab the gummed flap on the interior surface, covering the full adhesive strip. A whole paper punch or cut-out from the moistened area can also be submitted if destructive sampling is approved.
• Stamps: Self-adhesive stamps carry no amylase; only lick-and-stick stamps are relevant. Swab the adhesive side carefully to avoid damaging the philatelic item.
• Order of examination: Take biological samples before fingerprinting reagents are applied. Many fingerprint chemicals denature proteins and can destroy amylase activity or interfere with immunoassays.

Sensitivity studies have shown that Phadebas can detect amylase in saliva diluted to approximately 1:1000 or 1:2000, which means even a very small stain contains detectable enzyme. This is good for catching low-volume deposits, but the same sensitivity means that sources of amylase other than saliva are also picked up at low concentrations.

RSID-Saliva improves specificity by targeting the human salivary isoform, but no immunoassay is perfectly specific in all realistic casework conditions. Vaginal fluid can contain salivary amylase if oral contact occurred, which is not a contamination artifact but a genuine reflection of how secretions mix in sexual assault cases. Reporting needs to acknowledge that a positive RSID result on vaginal swabs does not automatically mean oral contact: cross-contamination at collection is another explanation.

A negative result from Phadebas and RSID-Saliva does not prove saliva is absent. Degradation by UV light, extreme temperature, cleaning products, or simple passage of time can reduce amylase below the detection threshold. As with all trace evidence, the absence-of-evidence principle applies: failure to detect is not proof of non-deposition.

A well-constructed saliva report distinguishes between levels of inference. The first level is analytical: amylase activity was detected at a concentration consistent with a salivary deposit. The second level is inferential: this is consistent with saliva being present on the exhibit. The third level is contextual: given the exhibit type and case circumstances, oral contact is one explanation.

Courts in the UK and elsewhere have accepted amylase evidence when properly qualified. The key is honesty about the specificity of the assay used. A Phadebas-only positive should be reported as indicating the presence of alpha-amylase, with the note that salivary origin is the most likely but not the exclusive explanation. An RSID-Saliva positive supports a narrower claim of human salivary amylase detected, which is a stronger foundation for arguing oral contact.