Column agglutination (gel card) technology

Principle

• Reactions take place in microtubes containing a dextran-acrylamide gel (e.g. Bio-Rad ID) or glass beads (e.g. Ortho BioVue), which act as a sieve.
• During controlled centrifugation, agglutinated cells are trapped at the top or within the column; unagglutinated cells pass through and form a button at the bottom.
• Columns may be neutral (no reagent), contain specific antisera (anti-A, anti-B, anti-D) or anti-human globulin.

Specimen

• EDTA blood; plasma free of fibrin. Red cells are prepared as a low-concentration suspension (commonly about 0.8–1%) in the manufacturer's diluent.

Procedure

• Pipette red cells (and plasma where needed) into the reaction chamber above the gel in the volumes specified.
• For IAT, incubate the card at 37 °C (commonly 15 min).
• Centrifuge in the dedicated card centrifuge for the specified time (about 10 min).
• Read from front and back of the card.

Results

• 4+: solid band at the top of the gel.
• 3+: most agglutinates in the upper half.
• 2+: agglutinates spread throughout the column.
• 1+: small agglutinates in the lower half, some cells at the bottom.
• Negative: all cells form a button at the bottom.
• Mixed field: band at the top plus a button at the bottom.

Quality control and pitfalls

• No washing step is needed for AHG cards because the gel separates cells from unbound plasma globulins.
• Fibrin or incompletely clotted serum, bubbles, dried or damaged gel, and wrong cell concentration cause false reactions.
• Haemolysis may be less obvious than in tubes.
• Check cards before use and run daily positive and negative controls.

Clinical use

• ABO/D grouping, antibody screening and identification, crossmatch, DAT and phenotyping.
• Advantages: standardised, objective, stable reactions, small sample volume, suitable for automation.