ABO Typing on Stains: Absorption-Elution and Inhibition

Standard forward ABO grouping tests a suspension of red cells against known antisera. The cells agglutinate when antibody bridges adjacent cells bearing the matching antigen. Reverse grouping tests the serum against known cells. Both directions depend on intact, suspended cells with intact surface antigen at normal density.

A dried bloodstain provides none of this. The red cells rupture as they dry, releasing haemoglobin and leaving membrane fragments. There is no cell suspension possible. The serum proteins are denatured and spread through the substrate. The ABO antigens do remain, associated with membrane glycolipid and glycoprotein fragments, but they are fixed in a substrate matrix rather than floating in saline. Antibody can still bind to them, but there is no cell bridging, and therefore no agglutination.

The absorption-elution method was refined for forensic use by several groups including the UK Home Office Forensic Science Service laboratories through the 1960s and 1970s, and became the standard technique for most Western forensic laboratories. Its appeal is sensitivity: it detects antigen by the presence of antibody captured onto the stain, not by agglutination of intact cells, so very small amounts of antigen are detectable.

1. 1. Absorption
   A small portion of the stain (typically a few milligrams of stained substrate) is incubated with an antiserum of known specificity, anti-A or anti-B (or both, in separate reactions). The antiserum is added in a defined volume and at a defined concentration. Antibody binds to any matching antigen on the stain material. The incubation runs at 4 degrees Celsius for optimal binding.
2. 2. Washing
   After incubation, the stain substrate is washed repeatedly with cold saline to remove all antibody that did not bind specifically to antigen. This step is critical: incomplete washing leaves free antibody that would produce a false-positive result in the elution step.
3. 3. Elution
   The washed substrate is heated in a small volume of saline, typically at 56 degrees Celsius for several minutes. Heat breaks the antigen-antibody bonds and releases the antibody into the saline supernatant. This eluate is then removed from the substrate.
4. 4. Testing the eluate
   The eluate is added to indicator red cells of the appropriate group (A-positive cells for the anti-A test; B-positive cells for the anti-B test). If agglutination occurs, the eluate contains the eluted antibody, confirming that the stain carried the corresponding antigen. No agglutination means no antibody was bound, so the antigen was absent.

Controls are not optional in this procedure. A positive control uses a substrate known to carry the relevant antigen. A negative control uses a substrate known to lack it, and also the same substrate without any stain, to detect non-specific antibody binding to the substrate itself. Substrates like cotton, wool, and denim vary in their background antibody binding, and failing to run substrate controls is a major quality failure.

In the absorption-inhibition method, the logic runs in the opposite direction from absorption-elution. Rather than asking what antibody can we capture from the stain, the analyst asks how much of a known antibody does the stain neutralise. The stain extract is mixed with a measured volume of antiserum of known titre. If the stain contains the corresponding antigen, some or all of the antibody will be absorbed out of the antiserum by binding to stain antigen. The residual antibody activity is then measured against indicator cells.

A significant fall in titre, compared to a control tube with no stain extract, is a positive result indicating antigen was present. The extent of inhibition can be quantified: a one-tube titre drop is weak evidence; a four-tube or greater drop is strong. The agglutination-inhibition variant uses the same principle on saliva or other body-fluid extracts and was the primary method for typing secretion stains, since secreted ABO antigens (blood-group substances) are in soluble form and cannot be typed by absorption-elution.

Mixed agglutination was developed for forensic stain work by researchers including Dodd and Hunter in the early 1960s, building on earlier serological methods. It was designed for situations where the stain area is so small, perhaps a single fibre or a speck on an instrument, that absorption-elution cannot be run. The technique works directly on the stain surface rather than requiring extraction.

The procedure: the stained material is placed on a glass slide and incubated with dilute antiserum (anti-A or anti-B). After washing, indicator red cells are added. If the stain carries the matching antigen, antibody bridging occurs between the stain surface antigen and the indicator cells, forming a mixed aggregate or rosette visible under a microscope at 40-100 times magnification. The characteristic appearance is a ring or clump of indicator cells adhering to the stain material.

Three factors degrade stain typing results: age, dilution, and substrate contamination. Each operates through a different mechanism and requires a different analytical response.

• Age and bacterial degradation: moist conditions promote bacterial growth in a stain. Many bacteria produce glycosidases that cleave the terminal sugars from ABO antigens, converting A and B antigens back toward H. A six-month-old stain stored in a damp location can give a false-negative or an apparent group O result even from a genuine group A or B donor. Dry storage dramatically slows this degradation.
• Dilution and small stain volume: antigen density falls proportionally with dilution. A heavy bloodstain on cotton provides abundant antigen; a trace quantity on a hard surface may fall below the detection threshold for any of these methods. Sensitivity varies: absorption-elution is generally the most sensitive for fabric stains; mixed agglutination can work on smaller areas but with greater interpretive uncertainty.
• Substrate and contaminant interference: certain plant materials, particularly some legumes and grasses, contain lectins that bind to ABO antigens or to antisera, producing spurious results. Soil microorganisms on an outdoor stain bring the same bacterial enzyme problem. A urine-contaminated semen stain on fabric adds a second donor's ABO substances, complicating interpretation if the two donors have different groups.

The analyst's obligation in these situations is to document the stain condition, note the specific challenges present, and qualify the result accordingly. A statement such as 'the stain is consistent with group A' is appropriate when the absorption-elution result is positive. 'The stain could not be typed' is appropriate when controls fail or the substrate background is too high. 'The stain is group O' is never appropriate when it may be a degraded group A or B stain, unless bacterial degradation has been excluded by additional testing.

The sensitivity of absorption-elution depends on the antibody titre used, the volume of eluate, and the reactivity of the indicator cells. Published studies from UK forensic laboratories in the 1970s and 1980s established that positive results could be obtained from stains as small as 1 milligram of dried blood on cotton fabric. In practice, the threshold varies: group A stains are typically more easily typed than group B because anti-A sera were historically of higher titre; group O stains require anti-H lectin (from Ulex europaeus) and are more substrate-sensitive.

Quality assurance in historical serology casework was variable. Modern forensic scientists reviewing older reports need to check whether the original analyst ran substrate controls, whether multiple independent tests were performed for each stain, and whether the reagents used were standardised and tested before use. Absent these records, it is often impossible to know whether a negative result reflected genuine absence of antigen or a methodological failure.