15.4 Molecular, Solid-Phase, Column, ELISA, and Related Methods

15.4 Molecular, Solid-Phase, Column, ELISA, and Related Methods

Quick Answer: Column agglutination (gel) traps agglutinates in the matrix; mixed-field is a dual band (line in the gel plus a pellet). DAT-positive cells can cause extra gel reactions. Solid-phase red-cell adherence uses IgG-coated indicator cells: a monolayer is positive, a button is negative. It is highly sensitive for Kidd and is wrecked by hemolyzed samples. ELISA/EIA is how donor viral markers (and some platelet serology) are read. Molecular immunohematology uses SNP / HEA arrays and RHD variant testing to predict a phenotype. Neutralization: saliva (ABH, Lewis), hydatid / P1 substance (anti-P1), plasma (Ch/Rg), urine (Sda). Predicted types do not replace serology when the genotype is incomplete or you still need an antibody screen.

The June 9, 2026 outline lists these methods under IV.C. Tube IAT is still the reference language of the exam. These platforms are how most laboratories actually detect Kidd, resolve a weak D, and prove anti-Sda is anti-Sda.

Column agglutination (gel)

A gel card (or glass-bead column) is a microtube filled with dextran-acrylamide gel, often preloaded with anti-A, anti-D, or anti-IgG. You add a measured cell suspension ± plasma, incubate if the card requires it, and centrifuge.

• Unagglutinated cells travel through the gel and form a button at the bottom — negative.
• Large agglutinates are trapped at the top — 4+.
• Smaller agglutinates stall throughout the gel — 1+ to 3+.
• Mixed-field: a band of agglutinated cells in the gel plus a pellet of free cells at the bottom. That dual population is transfusion, chimerism, stem-cell transplant, or a subgroup such as A3. Gel makes mixed-field easier to see than a shaken tube.

Most IAT gel cards are anti-IgG only. They will not detect a complement-only antibody. They also skip the tube wash step because unbound IgG is excluded by the gel — that is the operational advantage, not a reason to skip check-cell logic on a tube DAT.

DAT interference: IgG-coated patient cells can agglutinate in cards that contain AHG or can look “extra positive” in reverse-grouping or antibody-detection cards. Wash, use a saline card, or move the discrepancy to tube. Do not invent a new ABO type because a DAT-positive sample looked mixed-field in gel.

Gel reactions are stable (you can photograph the card). Sensitivity is high but not identical to PEG tube or solid phase; a method-dependent antibody can be gel-positive and tube-negative, or the reverse. Identify it. Do not call the other method “wrong.”

Solid-phase red-cell adherence

Solid-phase wells are coated with red-cell membranes or dried reagent cells. Patient plasma incubates in the well. Unbound immunoglobulin is washed away. Indicator cells — red cells coated with anti-IgG — are added and centrifuged.

That readout is the opposite of a gel pellet. Memorize monolayer = positive before the item tries to flip it.

Solid-phase is very sensitive, and it is famous for catching Kidd antibodies that a LISS tube screen missed. The same sensitivity means it lights up warm autoantibodies you might have lived with in a less sensitive method.

Hemolyzed samples (and often lipemic or grossly icteric ones) interfere — free stroma and hemoglobin make invalid monolayers or unreadable wells. Recollect. Do not force a solid-phase interpretation on a cherry-red EDTA.

Solid-phase does not display mixed-field the way gel does. It also does not replace a panel; many laboratories reflex a solid-phase positive to gel or tube identification.

ELISA / EIA

Enzyme-linked immunosorbent assays bind antigen or antibody to a plate, add an enzyme-conjugated detector, and read a color / optical-density change.

In the transfusion service you meet ELISA as:

• Donor infectious-disease serology: HBsAg, anti-HBc, anti-HCV, HIV antigen/antibody, anti-HTLV, T. cruzi, and related markers (NAT is a different platform).
• Some platelet serology: support for anti-HPA investigations and platelet-antibody screens, often alongside MAIPA or other capture assays.

ELISA is not how you routinely identify anti-Jka. Do not turn every “EIA” stem into an antibody screen. If the stem is a donor marker or a platelet-antibody workup, EIA is in play. If the stem is a red-cell alloantibody, you are still in gel, solid-phase, or tube.

Molecular immunohematology

HEA (human erythrocyte antigen) genotyping — usually a SNP array or targeted PCR — predicts the red-cell phenotype from DNA: Fy, Jk, MNS, Kell, Diego, Dombrock, Lutheran, and the rest of the chip’s menu. RHD variant resolution distinguishes common weak D types 1, 2, and 3 (manage as D-positive for transfusion and RhIG in current guidance) from partial D (D-negative units and RhIG).

Order molecular typing when serology is lying or impossible:

• Recently transfused and cell separation failed
• DAT-positive cells that type as pan-positive with typing sera
• Chronically transfused patients (sickle cell disease) who need an extended match
• Discrepant or unexplained weak / partial D
• A high-prevalence workup where the predicted type points at the system

Limitations — serology is still required:

• The array predicts only the alleles it was built to see. A novel hybrid, an unlisted silencing mutation, or a cis product can yield a predicted-positive, serologic-negative (or the reverse) type.
• ABO molecular is allele-rich. Serologic ABO remains the primary type for transfusion.
• DNA does not detect an alloantibody. You still need a screen, a panel, and a crossmatch strategy.
• A predicted Fy(b−) that is actually the GATA-1 erythroid silence is usually on the chip — that is a win — but you still honor antibodies, not only genotypes.
• Fetal RHD from cell-free DNA guides RhIG and monitoring; it does not type the mother’s antibody.

Neutralization and inhibition

Soluble antigen can inhibit the matching antibody. A true neutralization abolishes or sharply weakens the reaction compared with a dilution control (same volume of saline or inert protein). If the dilution control also went weak, you only diluted the antibody.

Neutralization is a confirmation tool, not a screen. You already think you have anti-P1, anti-Sda, or anti-Ch; the substance proves it. Substances can be dirty. Hydatid fluid is not an excuse to skip a panel.

When serology still has the last word

Use the platform that answers the question:

• Gel when you need a stable IAT or a mixed-field picture.
• Solid-phase when you need Kidd-level sensitivity and the sample is not hemolyzed.
• ELISA for viral markers and selected platelet serology.
• HEA / RHD genotyping when the phenotype is mixed, coated, or variant.
• Neutralization when a soluble substance exists for that specificity.

You still need serology to identify the antibody in the plasma, to type ABO for transfusion, to read a DAT, and to crossmatch against the actual unit. A perfect SNP printout of Jk(a−b+) does not tell you whether today’s anti-Jka has fallen below detectability.

Exam traps

• Calling a gel bottom button a positive solid-phase result. Gel pellet = negative; solid-phase button = negative; solid-phase monolayer = positive.
• Ignoring mixed-field in gel as “dirty centrifugation.”
• Running solid-phase on a hemolyzed sample and trusting the monolayer.
• Treating an HEA predicted type as a license to skip the antibody screen or to override a clear serologic ABO.
• Claiming saliva from a nonsecretor will inhibit anti-H.
• Skipping the dilution control and calling weaker reactions neutralization.