Extra Reactivity in Serum Grouping Tests

Extra reactivity in serum grouping tests ("extra antibodies") involves unexpected positive results in the reverse serum group. There are four causes of extra antibodies as illustrated in Table 4-7:

Table 4-7. "Extra Antibodies."

Examples	anti-A	anti-B	A₁ cells	B cells	Tentative group
#1	4+	--	1+	4+	A
#2	4+	4+	2+	--	AB
#3	--	4+	4+	1+	B

Anti-A₁ in A₂ or A₂B people: examples #1 and #2 illustrate the presence of anti-A₁. The autocontrol (not shown) would be negative.
Discussion and resolution of this discrepancy is provided earlier in the section on A₁ and A₂ subgroups.
Irregular IgM Alloantibodies: All three examples could represent the presence of irregular IgM alloantibodies such as anti-M, -N, -Le^a, -Le^b, or -P₁. The A₁ (or B cells) may be agglutinating because they are positive for the corresponding antigen. The autocontrol (not shown) would be negative.
Resolution:
Rouleaux: providing both cells in the reverse grouping show agglutination (examples #1 and #3), the discrepancy could be due to rouleaux. The autocontrol (not shown) would be positive.

Causes: Rouleaux is a type of false agglutination caused by an increase in serum globulins. This can occur in diseases such as multiple myeloma or macroglobulinemia or can be caused by infusion of macromolecular substances such as dextran or polyvinylpyrollidone (PVP), which are used as blood volume expanders.

Typical reaction pattern:

Resolution:
Autoanti-I: Many people have a harmless autoanti-I that is IgM and reacts best at 4°C. The harmless autoanti-I of most people will not react above 10° - 15°C, but some people have an autoanti-I that can react at RT and cause unexpected agglutination in both cells of the reverse serum group (examples #1 and #3).

Typical reaction pattern:

Resolution:

Extra Antibodies