12.2 Adsorptions, Elutions & Titrations

Adsorption Studies: Removing Antibody to Reveal What's Underneath

Adsorption (also called absorption) means incubating serum or plasma with antigen-positive red cells so a specific antibody binds to (is removed by) those cells, leaving the residual serum available for further testing. It is the core technique for separating a masking antibody - almost always a warm autoantibody - from an underlying alloantibody that needs to be identified before transfusion.

Autoadsorption

Autoadsorption uses the patient's own red cells to remove autoantibody from the patient's own serum. Because the patient's cells are already coated with autoantibody, they must first be stripped with a reagent such as ZZAP (a combination of dithiothreitol and a proteolytic enzyme, covered in Section 12.3) before they can be used as adsorbing cells. After ZZAP treatment and washing, the patient's own stripped cells are mixed with the patient's serum; any autoantibody re-adsorbs onto the patient's antigens, and the adsorbed serum is then tested against a standard panel to reveal any alloantibody that had been hiding underneath the autoantibody's broad reactivity.

Critical limitation: autoadsorption is invalid if the patient has been transfused within the preceding three months. Circulating donor cells mixed with the patient's own population mean the "autologous" adsorbing cells are not purely autologous, and a coexisting alloantibody could be adsorbed out along with the autoantibody - hiding exactly what you are trying to find.

Alloadsorption (differential adsorption)

When autoadsorption is invalid (recent transfusion) or autologous cells are unavailable (severe anemia, recent hemolysis), alloadsorption substitutes a panel of reagent red cells of known phenotype - classically three cells selected to represent the common Rh haplotypes: R1R1, R2R2, and rr. Serum is adsorbed separately with each of the three cells, and the pattern of antibody removed (or not removed) by each cell is compared, using the same rule-out logic applied to antibody-panel interpretation. Because none of the three adsorbing cells is genetically identical to the patient, alloadsorption can never remove a strong autoantibody as completely as true autoadsorption, so results should be interpreted alongside titers and the differential pattern rather than treated as an absolute negative/positive call.

Worked example

A patient with warm autoimmune hemolytic anemia shows 2+ to 3+ reactions with every cell on a standard antibody panel - a classic panreactive pattern. Because the patient was transfused six weeks earlier, autoadsorption is invalid, so the technologist performs alloadsorption with three reagent cells: R1R1, R2R2, and rr. After adsorption, testing the adsorbed sera against a full panel shows that only Jk(b+) cells still react, while Jk(b-) cells are negative across all three adsorptions. This pattern - reactivity confined to one specific antigen after the broad autoantibody has been removed - identifies an underlying anti-Jkb that had been masked by the autoantibody.

Elution: Recovering Antibody From Sensitized Cells

Elution is the reverse operation: dissociating antibody that is already bound to (coating) a red cell membrane so the released antibody (the eluate) can be tested against a panel to determine its specificity. Elution answers the question "what antibody is coating this red cell?" - exactly the question raised by a positive direct antiglobulin test (DAT) with unknown cause.

When elution is used

• Working up a positive DAT of unknown specificity (autoimmune hemolytic anemia, drug-induced antibody, delayed hemolytic transfusion reaction).
• HDFN (hemolytic disease of the fetus and newborn) investigation - eluting antibody from the newborn's DAT-positive cells identifies the maternal antibody responsible.
• Hemolytic transfusion reaction workup - eluting antibody from a post-transfusion sample to confirm which donor-antigen specificity caused the reaction.

Elution methods

Quality control rule: every eluate must be tested alongside a last-wash control (the supernatant from the final wash before elution). If the last-wash control reacts as strongly as the eluate itself, residual unwashed antibody - not truly membrane-bound antibody - is being detected, and the elution result is invalid.

Titration Studies: Quantifying Antibody Strength

Titration measures how much antibody is present by testing serial twofold (doubling) dilutions of serum against a constant antigen-positive reagent cell. The titer is reported as the reciprocal of the highest dilution that still shows a defined minimum reaction (commonly 1+ macroscopic agglutination).

Clinical uses

• Monitoring maternal alloimmunization in pregnancy - serial anti-D (or other clinically significant antibody) titers track whether HDFN risk is rising. A commonly cited critical titer for anti-D is 16; reaching or exceeding the critical titer, or a fourfold rise between specimens, triggers referral for middle cerebral artery Doppler monitoring.
• ABO-incompatible transplantation - pre- and post-transplant ABO antibody titers guide desensitization and rejection monitoring in solid-organ and HPC transplant.
• Distinguishing clinically significant vs. insignificant cold antibodies - a very high-titer cold agglutinin is more likely to cause clinically relevant hemolysis or interference than a low-titer one.

Titration score

Because a titer alone is a blunt endpoint (it only changes in whole doubling steps), many labs also calculate a titration score: each dilution's reaction strength is converted to points (commonly 4+=12, 3+=10, 2+=8, 1+=5, +/-=2, negative=0) and summed across all dilutions tested. The score can rise or fall between two specimens even when the titer endpoint itself stays the same, making it a more sensitive trend indicator.

Worked example

A prenatal patient's anti-D is tested in serial twofold dilutions with the following results:

The last dilution showing at least a 1+ reaction is 1:16, so the reported titer is 16. Because this meets the commonly used critical titer of 16, this result alone is a signal for closer fetal surveillance - though the more important comparison is against the same patient's prior specimen tested in parallel, side by side, using the same technique and reagent red cells, since titers are not reliably comparable across different runs, reagent lots, or laboratories.