The antibody screen

Try first

A two-cell antibody screen is nonreactive. Cell 1 is Jk(a+b+) and cell 2 is Jk(a−b+). Could a weak anti-Jk^a still be present?

No, because cell 1 carries Jk^a and did not react

Keep going to see the reasoning.

No, because Kidd antibodies react at immediate spin, which the screen would show

Keep going to see the reasoning.

Yes, because cell 1 carries a single dose of Jk^a, which a weak Kidd antibody can miss

Right. Keep going to follow the reasoning.

Only if the autocontrol is reactive

Keep going to see the reasoning.

I don't know yet

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Group O screening cells

The antibody screen tests the patient's plasma against two or three group O reagent red cells with known antigen profiles. Group O cells lack A and B, so the patient's expected anti-A and anti-B do not react with them. A reaction indicates an unexpected antibody.^1,2

Across the lot, the cells carry D, C, E, c, e, K, k, Fy^a, Fy^b, Jk^a, Jk^b, Le^a, Le^b, P1, M, N, S and s. Each cell is tested separately, and the lot's antigen profile lists what each cell carries.³

What a negative screen means

A nonreactive screen shows no antibody detectable against the antigens on those cells, under those test conditions.^1,4 It can miss:

a weak antibody, or one that has fallen below detection since it formed
an antibody that shows dosage, when the cells carry only a single dose of its antigen
an antibody to a low-prevalence antigen that the cells lack and a donor unit carries

Unit selection follows the patient's antibody history even when today's screen is nonreactive. A recorded clinically significant antibody requires antigen-negative red cells and an antiglobulin crossmatch.^1,2

How old a specimen can be

Transfusion and pregnancy can stimulate a new antibody within days. Federal rules require a specimen less than 3 days old when the patient was transfused or pregnant in the preceding 3 months.⁵ The collection date is day 0, and the specimen expires at midnight on day 3. AABB standards apply the same limit when the history is unknown or uncertain.^1,2 A longer interval needs a reliable negative history and a defined facility policy.^2,6

Follow the bench demonstration

Sources for this step

References

Bloch EM, Campbell-Lee S, McKenna DH Jr, Montemayor-Garcia C, Schwartz J, Shaz B, Storry J, eds. Technical Manual. 22nd ed. AABB; 2026.
Association for the Advancement of Blood & Biotherapies. Fundamental Standards for Blood Collection and Transfusion. 2nd ed. AABB; 2025. Accessed September 27, 2026.
Testing of source material, 21 CFR §660.33 (2026). Accessed September 27, 2026. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-F/part-660/subpart-D/section-660.33
Immucor, Inc. Panoscreen Reagent Red Blood Cells Package Insert. US Food and Drug Administration. Accessed September 27, 2026.
Compatibility testing, 21 CFR §606.151 (2026). Accessed September 27, 2026. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-F/part-606/subpart-C/section-606.151
Association for the Advancement of Blood & Biotherapies. Standards for Blood Banks and Transfusion Services. 35th ed. AABB; 2026. Accessed September 27, 2026.

Watch one

Read each decision and its explanation.

Tobias Staines, 66, is typed and screened before hip surgery. His record shows anti-Jk^a identified 2 years ago. Today's three-cell screen reads:

The check cells agglutinated in every tube. What does the screen show, and which red cells does he receive?

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b
1	+	+	0	0	+	+	+	+	0	+	+	+	0	+	+	+	0	+
2	+	0	+	+	0	0	+	0	+	0	+	0	+	0	+	0	+	0
3	0	0	0	+	+	0	+	+	+	+	0	+	+	0	+	+	0	+

Check validity first. Every cell is 0 at 37 °C and the antiglobulin phase, and the check cells agglutinated.
A negative tube reading counts only when the check cells show the antihuman globulin was active.
Read the Kidd antigens. Cell 3 is Jk(a+b−), a double dose of Jk^a, and it is nonreactive.
A weak Kidd antibody is most likely to react with a cell carrying a double dose.
Interpret the screen. No anti-Jk^a is detectable today.
Kidd antibodies can fall below detection and return within days of re-exposure.
Check the record. The anti-Jk^a identified 2 years ago remains on it.
A recorded clinically significant antibody requires antigen-negative units even when today's screen is nonreactive.
Select Jk(a−) red cells and crossmatch them through the antiglobulin phase.
A history of a clinically significant antibody excludes an immediate-spin or electronic crossmatch.

The screen is reported as nonreactive. He receives Jk(a−) red cells with an antiglobulin crossmatch, because of his history of anti-Jk^a.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A valid antibody screen is nonreactive. Which interpretation is supported?

The recipient has never formed a red-cell antibody

Incorrect. Historical records remain necessary because past antibodies can become undetectable.

Every donor unit will be serologically compatible

Incorrect. A donor may carry an antigen absent from the screening cells, and other incompatibility causes remain possible.

No antibody was detected against these cells under these test conditions

Correct. The conclusion is limited by reagent antigen expression, antibody concentration, and method sensitivity.

Hint

A screen tests one set of reagent cells under one set of conditions.
Ask which options claim something about cells or times the screen never tested.

Review Unexpected-antibody detection

Problem 2 of 3 · Hints available

Why are the reagent red cells in an antibody screen group O?

Group O cells carry more Rh antigen than cells of other groups

ABO group does not affect Rh antigen density. Each cell's Rh antigens are listed on the lot profile.

Group O cells lack A and B, so expected anti-A and anti-B cannot react with them

Any reaction with group O cells then indicates an unexpected antibody in a patient of any ABO group.

Only group O donors carry all the antigens the lot must include

Donors of any group can carry these antigens. The group matters because of the patient's expected ABO antibodies.

Group O is the most common donor group, so the screen matches most units

The screen looks for antibodies in the patient's plasma. It makes no prediction about any donor unit.

Hint

Ask what every patient's plasma may contain besides unexpected antibodies.
Recall the expected antibodies of a group A patient and of a group B patient.

Review Unexpected-antibody detection

Problem 3 of 3 · On your own

A patient was transfused 5 weeks ago. Her specimen was collected on Monday at 06:00. On Thursday at 10:00 a crossmatch is requested on that specimen. Can it be used?

Yes, until midnight on Thursday

Monday is day 0, and Thursday is day 3. The specimen expires at midnight on day 3.

No, it expired at 06:00 on Thursday, 72 hours after collection

The limit counts calendar days. Monday is day 0, and the specimen lasts until midnight on day 3.

No, a patient transfused in the past 3 months needs a specimen drawn the same day

The rule allows a specimen less than 3 days old. A same-day specimen is not required.

Yes, for 10 days, because her screen was nonreactive

Her transfusion 5 weeks ago sets the 3-day limit. A nonreactive screen shows only the antibodies present when the specimen was drawn.

Review Identity, specimen, and history

Use it

Read the case evidence, then make each decision. The step is complete without it.

Hollis Sampson, 58, a long-haul trucker, MRN 7730418, arrives unresponsive after a highway crash. No transfusion history can be found.
A type and screen was collected on Sunday at 14:00.
On Friday, surgery asks for 2 red cell units on that specimen.
The facility's policy allows a specimen up to 14 days old for a patient with a reliable negative history.

Decision 1 of 3

Can the Sunday specimen be used?

Yes, under the 14-day policy

The longer interval needs a reliable negative history. His history is unknown, so the 3-day limit applies.

Applied a longer sample interval when history was unknown

A patient transfused or pregnant in the past 3 months can form a new red-cell antibody within days, and an older sample would miss it. Federal rules require a sample less than 3 days old for these patients, and AABB standards apply the same limit when the history is unknown or uncertain. A longer interval needs a reliable negative history and a defined facility policy.

Yes, because he has no recorded transfusion

A reliable negative history comes from records or a reliable account. He has neither, so his history is unknown.

No, it expired at midnight on Wednesday, so a new specimen is collected

With an unknown history the 3-day limit applies. Sunday is day 0, and the specimen expired at midnight on Wednesday, day 3.

Review Identity, specimen, and history

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b	37	AHG
1	+	+	0	0	+	+	+	+	+	+	+	+	+	+	0	+	0	+	0	0
2	+	0	+	+	0	0	+	+	0	0	+	0	+	0	+	+	+	0	0	0

Decision 2 of 3

The new specimen's two-cell screen reads as shown. Which antibody could it most easily miss?

A weak anti-E

Cell 2 is E+e−, a double dose of E, and it is nonreactive. A weak anti-E would most likely react with it.

A weak anti-D

Both cells carry D and both are nonreactive. Anti-D would react with either.

A weak anti-K

Cell 1 carries K and is nonreactive. Kell antibodies do not depend on a double dose to react.

A weak anti-Jk^a

Only cell 1 carries Jk^a, and it is Jk(a+b+), a single dose. A weak Kidd antibody can miss a single dose.

Review Unexpected-antibody detection

Decision 3 of 3

The new specimen was collected on Friday at 09:00. Until when can it be used for crossmatching?

Until 09:00 on Monday

The limit counts calendar days. Friday is day 0, and the specimen lasts until midnight on day 3.

Until midnight on Monday

Friday is day 0, Saturday day 1, Sunday day 2 and Monday day 3. It expires at midnight on Monday.

Until midnight on Sunday

That counts Friday as day 1. The collection date is day 0.

Review Identity, specimen, and history

His transfusion history is unknown, so the 3-day limit applied to both specimens. With no antibody record, today's nonreactive screen, with only a single dose of Jk^a on its cells, is all that is known about his antibodies.

For reference

Sources checked September 27, 2026

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The antibody screen

Try first

Get the idea

Group O screening cells

What a negative screen means

How old a specimen can be

Watch one

Your turn

Use it

For reference

The rest of this step has a short briefing, a demonstration at the bench, 3 practice problems and a short case.

Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b
1	+	+	0	0	+	+	+	+	0	+	+	+	0	+	+	+	0	+
2	+	0	+	+	0	0	+	0	+	0	+	0	+	0	+	0	+	0
3	0	0	0	+	+	0	+	+	+	+	0	+	+	0	+	+	0	+

Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b
1	+	+	0	0	+	+	+	+	0	+	+	+	0	+	+	+	0	+
2	+	0	+	+	0	0	+	0	+	0	+	0	+	0	+	0	+	0
3	0	0	0	+	+	0	+	+	+	+	0	+	+	0	+	+	0	+

Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b
1	+	+	0	0	+	+	+	+	0	+	+	+	0	+	+	+	0	+
2	+	0	+	+	0	0	+	0	+	0	+	0	+	0	+	0	+	0
3	0	0	0	+	+	0	+	+	+	+	0	+	+	0	+	+	0	+