13.1 Blood Grouping Tests

13.1 Blood Grouping Tests

Quick Answer: Forward ABO tests the patient's red cells with anti-A and anti-B. Reverse ABO tests the patient's plasma against A1 and B reagent cells. The two must agree before you report a type. D is typed with anti-D, usually at immediate spin. Serologic weak D (IAT/AHG) is required on allogeneic donors and on newborns of D-neg mothers; it is not routinely required on patients. Tube, gel/column, and solid-phase are all valid once validated. Do not report an ABO discrepancy — if red cells are needed now, issue group O. Neonates do not get a reverse type because they have not made isoagglutinins.

The June 9, 2026 BB outline parks this work at IV.A.1. Chapter 6 already taught the sugars and the named discrepancy buckets. Chapter 7 already taught weak D versus partial D. This section is the bench procedure: what you set up, what you control, when you stop, and what the computer is allowed to remember. Clerical ABO mistakes still kill patients. The serology is how you refuse to be the next one.

Forward type is the antigen; reverse type is the antibody

Forward (cell) typing mixes patient or donor red cells with licensed anti-A and anti-B. Many cards also carry anti-A,B, which is useful for catching some weak subgroups (Ax) that monoclonal anti-A can miss. Modern reagents are monoclonal. A positive reaction means that antigen is on the cell. Grade 0, w+, 1+, 2+, 3+, 4+ the same way you grade everything else — do not invent a fifth scale for ABO.

Reverse (serum) typing mixes patient or donor plasma or serum with licensed A1 and B reagent red cells. A positive reaction means that antibody is in the plasma. In an immunocompetent adult the reverse type must mirror the forward type:

Group O plasma contains anti-A, anti-B, and anti-A,B. Group AB plasma contains neither. That empty reverse type is why AB plasma is the emergency plasma and why an extra reverse reaction in a supposed AB patient is a discrepancy, not a footnote. Use A1 cells, not random A cells, for the reverse type. An A2 patient with anti-A1 will agglutinate A1 cells and look like extra reverse reactivity. Chapter 6 already handed you Dolichos biflorus and A2 cells as the workup. Here the rule is simpler: forward and reverse must agree, and A2 with anti-A1 is still group A for red-cell transfusion if the antibody is not 37 °C-reactive.

Do not reverse-type neonates. Isoagglutinins are not reliably present for the first 3–6 months. A blank reverse type on a 3-day-old is expected, not a missing-reverse discrepancy. Assign ABO from a washed forward type. Cord cells must be washed free of Wharton jelly, which causes false extra forward agglutination that can make a group O infant look A, B, or AB.

EDTA is the usual specimen. Reject unlabeled, mislabeled, IV-diluted, and frankly hemolyzed tubes. Compare today's type with the historical type before you file it. A new result that disagrees with history is a discrepancy until you prove which one is right — wrong patient, HPC transplant, massive group-O transfusion, or a real subgroup.

Rh typing: immediate-spin D versus weak D

Routine D typing uses a monoclonal IgM or IgM/IgG blend anti-D and is read at immediate spin (or the gel/solid-phase equivalent). A clear 3–4+ is D-positive. A clear 0 is D-negative at IS.

Serologic weak D testing is an IAT / AHG type with anti-D. It asks whether IgG anti-D can coat cells that looked D-neg at immediate spin. United States practice, matching Chapter 7:

• Allogeneic donors: weak D is required. A donor who is D-neg at IS but D-pos at AHG is labeled D-positive. You must not put a weak-D unit in the D-neg fridge.
• Newborns of D-neg mothers: weak D is required. A weak-D infant counts as D-positive. The mother gets RhIG. Missing this is a preventable sensitization.
• Patients (including most prenatal patients): weak D is not routinely required. An IS D-neg patient may be treated as D-neg for transfusion and RhIG. CAP/AABB recommend RHD genotyping when a serologic weak D is found so types 1, 2, and 3 stop consuming D-neg inventory. Pending genotype, keep issuing D-neg red cells.

Immediate-spin typing detects IgM agglutination — ABO reagents and monoclonal anti-D. AHG typing detects IgG coating — weak D, some antigen typings, and any IAT phenotype. Do not call an AHG D type “just another immediate-spin.” The wash (tube) or the column (gel) is doing different work.

Rh control: high-protein anti-D and AB patients (whose cells can look sticky) need a negative reagent control (diluent or an inert control). If the control is positive, the D type is invalid. Spontaneous agglutination, a cold agglutinin, residual Wharton jelly, and a positive DAT can all make the control react. Fix the cause; do not “interpret around” a positive control.

Tube, gel/column, and solid-phase

All three platforms are acceptable if the laboratory has validated them.

• Tube: flexible, cheap, technician-dependent. Immediate-spin ABO/Rh, optional 37 °C readings, and AHG after washing. Reading is visual. Technique errors — underwash, overspin, dirty glassware, the wrong drop count — belong to you.
• Gel / column agglutination: red cells and antibody meet in a microtube with gel or glass beads. After centrifugation, agglutinated cells stay at the top or in a band; unagglutinated cells pellet. AHG gel cards do not require a wash step — the column traps unbound IgG. Reactions are graded 0 to 4+ by band position. Gel is sensitive to cold agglutinins and rouleaux. A mixed-field gel band after recent transfusion is information, not a card failure.
• Solid-phase: antigen or antibody is immobilized in a microwell. Indicator cells or a color readout show binding. Automated solid-phase antibody screens and some ABO/Rh platforms are AHG-equivalent methods. A solid-phase “positive” is adherence, not a classic button.

Pick the method your procedure names. Do not invent a hybrid (gel card spun like a tube, then “checked” with a stick). Daily reagent QC — anti-A, anti-B, anti-D, reverse cells — must react as expected before patient testing. Expired reagent is not a reagent. Reverse cells that have lost A or B antigen produce a false missing reverse and a wrong group AB.

Electronic records and the discrepancy hold

The LIS stores the forward type, reverse type, D type, and interpretation against two patient identifiers. A later computer crossmatch will demand two determinations of ABO — often one historical and one on the current specimen, or two current methods. Do not silently overwrite a historical type because today's card looks cleaner.

Forward and reverse must agree before you report an ABO type. Repeat on a washed specimen, check the label, check reagents, and then work the Chapter 6 buckets (extra reverse, missing reverse, extra forward, missing forward). Mixed-field forward types after recent group-O transfusion, an HPC transplant, or an A3/B3 subgroup are history questions, not typing errors. Record them. Do not “average” them into AB.

Do not guess. If the operating room cannot wait, issue group O red cells (O-neg for women of childbearing potential when D is also unknown) and AB plasma. Complete the type as soon as the workup allows, then switch to type-specific. The hold is the test. Reporting a guessed type is how ABO-incompatible blood leaves the fridge.

Exam traps

• Reverse-typing a neonate and calling a blank reverse a discrepancy.
• Skipping weak D on a donor or on the newborn of a D-neg mother.
• Doing weak D on every patient as if it were required, then labeling a partial-D patient D-pos without genotype.
• Reporting a type while forward and reverse disagree.
• Issuing type-specific red cells during an unresolved discrepancy.
• Reading a high-protein anti-D on an AB patient without an Rh control.
• Treating gel or solid-phase as “not really AHG.” Column and solid-phase typing that use anti-IgG are AHG methods.