13.2 Antibody Detection

13.2 Antibody Detection

Quick Answer: The antibody screen uses 2–3 group O reagent red cells that, between them, express the major antigens. Prefer homozygous Jka, Fya, and S so dosage does not hide Kidd, Duffy, or MNS. AHG is required. Immediate-spin for unexpected antibodies is optional if you use a computer crossmatch. A 37 °C reading is method-dependent. LISS speeds uptake; PEG concentrates antibody. Gel and solid-phase screens are AHG methods. A positive screen goes to identification (Chapter 14). A positive autocontrol is not a DAT — do the DAT. A negative screen can still miss a low-prevalence antigen, an antibody below detection, or a heterozygous cell.

The June 9, 2026 outline splits compatibility testing (IV.A.2) into detection here and the crossmatch in 13.3. Antibody identification is the next chapter. This section is the yes/no gate: is unexpected antibody present, and can you trust a negative?

Screen cells: group O, coverage, dosage

Licensed screening cells are group O so ABO isoagglutinins do not agglutinate them. You cannot screen a group A patient against group A cells and call the result an unexpected-antibody test — that is just reverse typing.

A 2-cell or 3-cell set is acceptable. A typical 3-cell set is phenotyped as R1R1 (DCe/DCe), R2R2 (DcE/DcE), and rr (dce/dce) so D, C, c, E, and e are present on at least one cell and several Rh antigens are present as double-dose. Between the cells in the set you must have the antigens that matter for clinically significant antibodies: D, C, E, c, e, K, k, Fya, Fyb, Jka, Jkb, S, s, M, N, P1, Lea, Leb, and the other required markings on the antigram. One cell does not have to carry everything. The set does.

Homozygous expression is the dosage defense. Kidd, Duffy, Rh (C/c/E/e), and MNS (M, N, S, s) can react more weakly with single-dose cells. A heterozygous Jk(a+b+) screen cell can miss anti-Jka. Whenever the vendor can, the set should include Jk(a+b−), Fy(a+b−), and S+s− cells. K does not show reliable dosage the way Kidd does, but you still want a K+ cell in the set. Low-prevalence antigens (Kpa, Jsa, Lua, Wra, V, VS) will not be on every screen cell. That is a known hole, not a QC failure.

Read the antigram every time. Do not memorize last month’s cell 2.

Phases: IS optional, 37 °C method-dependent, AHG required

AHG (IAT) is required for unexpected-antibody detection. Clinically significant antibodies are almost all IgG. If you never get to antiglobulin, you are not screening.

Immediate-spin reading of the screen is optional for unexpected antibodies when the laboratory uses a validated computer crossmatch. IS still has a job in ABO (reverse type and IS crossmatch). It is no longer mandatory as a phase for anti-E, anti-K, or anti-Jka. Dropping IS on the screen is how many labs stopped chasing room-temperature anti-M, anti-Lea, anti-P1, and cold autoagglutinins that they were never going to honor.

A 37 °C reading is method-dependent. Some tube LISS procedures still read a 37 °C button before washing. PEG procedures often skip a 37 °C read because PEG can cause nonspecific aggregation. Gel and solid-phase incubate at 37 °C and then read the AHG endpoint; there is no separate “37 °C grade.” Hemolysis at any phase is a positive result, not “just complement.”

Enhancement, gel, and solid-phase

Tube IAT without enhancement is slow and relatively insensitive. Two enhancers still own the exam:

• LISS (low-ionic-strength saline) speeds antibody uptake. Incubation is short, typically about 10–15 minutes at 37 °C. A too-short LISS incubation is a classic way to weaken some anti-K. Follow the insert; do not “save time” by cutting LISS to 5 minutes.
• PEG (polyethylene glycol) removes water and concentrates antibody. It is more sensitive than LISS and also more generous with warm autoantibodies and nonspecific reactions. After incubation you wash thoroughly and add AHG; many PEG methods use anti-IgG rather than polyspecific AHG to cut complement noise.

Albumin is the older zeta-potential reducer. Know it exists; do not write a new procedure around 22% albumin in 2026.

Gel/column screens and solid-phase red-cell adherence screens are AHG methods. The card or well already contains anti-IgG (or an AHG-equivalent detection system). You do not “add Coombs later.” A gel screen that is negative has already asked the AHG question. Treating gel as an IS-only method is how you fail an item that asks which method meets the AHG requirement.

Positive screen, autocontrol, and DAT

A positive screen is not a diagnosis. It is an order for antibody identification (Chapter 14). Do not issue red cells on a positive screen with a shrug and an IS crossmatch.

Run an autocontrol (patient cells + patient plasma, same method as the screen) with the screen or as soon as the screen is positive.

• Positive screen, negative autocontrol: alloantibody. Go to a panel.
• Positive screen, positive autocontrol: autoantibody, recently transfused alloantibody coating circulating donor cells, drug, or a cold that the method also sees. Next test is a DAT, not a longer incubation.
• DAT asks whether the patient’s cells were coated in vivo (cells + AHG, no plasma added). Autocontrol asks whether the patient’s plasma can coat the patient’s cells in the tube. They are not interchangeable. Section 13.4 finishes the DAT. Here you only need the fork: positive AC → DAT → decide whether this is auto, allo, or both.

A recently transfused patient can have a positive DAT and a mixed-field autocontrol because alloantibody is coating donor cells still in the circulation. That is a delayed-serologic or delayed-hemolytic picture, classically Kidd, not “the patient has a warm auto so ignore the screen.”

Limitations you must recite

A negative screen means circulating antibody is below the method cutoff on those cells today. It does not mean:

• The patient was never immunized. Memory outlives titer (Chapter 10). Kidd is the evanescent prototype.
• Every antigen is covered. Low-prevalence antigens are usually absent from the set. Anti-Wra, anti-Kpa, and anti-Jsa are allowed to appear first as an incompatible crossmatch or a single extra panel cell.
• Dosage is impossible. A heterozygous Jka, Fya, or S cell can look negative with a weak antibody that would have reacted with a double-dose cell.
• The specimen is timeless. If the patient was transfused or pregnant in the last 3 months, use a specimen collected within 3 days of the planned transfusion so a newly arriving anamnestic antibody is not missed. Many labs apply the 3-day rule to everyone.

Exam traps

• Screening with non–group-O cells and calling ABO isoagglutinins “unexpected.”
• Treating a 2-cell screen as illegal. Two cells are allowed if coverage is complete.
• Using only heterozygous Jka/Fya/S cells and then blaming the patient when anti-Jka appears after transfusion.
• Saying IS is required for unexpected antibodies even when a computer XM is validated.
• Calling gel or solid-phase “not AHG.”
• Substituting autocontrol for DAT, or DAT for identification.
• Promising a negative screen means no antibody can ever appear on the crossmatch.