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Lattices, prozone and agglutination

16 min

Skills you'll build3 skills
  • Recognize antibody excess as a cause of weak agglutination
  • Recognize antigen excess as a cause of a falsely low immunoassay result
  • Read direct and inhibition agglutination using the assay's controls

At the bench: Review immunoassay controls and recognize an unreliable run

Try first

Try first

A rapid plasma reagin (RPR) test on serum from a man with a rash on his palms reads nonreactive undiluted, with a rough, grainy look. The run's controls read as expected. What do you do before reporting?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Lattice formation

A visible precipitate or agglutinate forms only when antibodies with at least two binding sites bridge multivalent antigen again and again. The ratio of antigen to antibody determines the size of the lattice.1,2

ZoneBalanceWhat the test shows
ProzoneAntibody excessSmall complexes. A reactive specimen can appear weakly reactive or nonreactive.
EquivalenceBalancedLarge lattices. The reaction is strongest.
PostzoneAntigen excessSmall soluble complexes. A specimen with high antigen can give a falsely low or nonreactive result.

Prozone in the RPR

In an RPR, antibody excess can give a weakly reactive or nonreactive result on undiluted serum, often with a rough, grainy appearance. Prozone is rare in screened populations and occurs mainly in primary and secondary syphilis. Some specimens needed dilutions of 1:256 or 1:512 before they reacted.3 When the clinical picture suggests syphilis and the undiluted serum is nonreactive, test a dilution series.3

The titer is the reciprocal of the highest dilution that still reacts. Every reactive RPR is reported with its endpoint titer and goes on to a treponemal test.3

The hook effect in sandwich assays

Extreme antigen excess saturates the capture and detection antibodies separately. Few sandwiches form, and the result is falsely low, even inside the measuring range. When a result is unexpectedly low for the setting, dilute the specimen and retest under the validated procedure. A hooked result increases after correction for the dilution.2,4

Agglutination formats

In direct and passive agglutination, clumping means the antibody or antigen being sought is present. In reverse passive agglutination, particles carry reagent antibody, and clumping means antigen is present.1

Inhibition formats are read in the opposite direction. Antigen or hapten in the specimen neutralizes a limited amount of reagent antibody, so the coated particles cannot clump. No agglutination is the positive result.1

A patient result is valid only when the run's positive and negative controls give their expected reactions.1

Follow the bench demonstration

Sources for this step
References
  1. Abbas AK, Lichtman AH, Pillai S, Henrickson S. Cellular and Molecular Immunology. 11th ed. Elsevier; 2025.
  2. Jacobs JFM, van der Molen RG, Bossuyt X, Damoiseaux J. Antigen excess in modern immunoassays: to anticipate on the unexpected. Autoimmun Rev. 2015;14(2):160-167. doi:10.1016/j.autrev.2014.10.018
  3. Papp JR, Park IU, Fakile Y, Pereira L, Pillay A, Bolan GA. CDC laboratory recommendations for syphilis testing, United States, 2024. MMWR Recomm Rep. 2024;73(1):1-32. doi:10.15585/mmwr.rr7301a1
  4. Ghazal K, Brabant S, Prie D, Piketty ML. Hormone immunoassay interference: a 2021 update. Ann Lab Med. 2022;42(1):3-23. doi:10.3343/alm.2022.42.1.3

Watch one

Read each decision and its explanation.

Serum from the man with the rash is tested as a twofold RPR dilution series. The reactive, weakly reactive and nonreactive controls give their expected patterns.

How do you read and report this RPR?

TestResultReference intervalFlag
RPR, undilutedNonreactive, rough and grainyNonreactiveNonreactive
RPR, 1:2Weakly reactiveNonreactiveNonreactive
RPR, 1:4ReactiveNonreactiveNonreactive
RPR, 1:8ReactiveNonreactiveNonreactive
RPR, 1:16ReactiveNonreactiveNonreactive
RPR, 1:32ReactiveNonreactiveNonreactive
RPR, 1:64ReactiveNonreactiveNonreactive
RPR, 1:128NonreactiveNonreactiveNonreactive

Specimen: H 4, L 6, I 1. Serum, RPR dilution series

  1. Check the controls: reactive, weakly reactive and nonreactive controls each gave their expected pattern.

    A patient result is valid only when the run's controls give their expected reactions.

  2. Read the undiluted tube: nonreactive with a rough, grainy look.

    Antibody excess forms small complexes, which look rough and grainy.

  3. Read down the series: weakly reactive at 1:2, then reactive from 1:4 through 1:64.

    Each dilution lowers the antibody and moves the reaction toward equivalence.

  4. Find the endpoint: 1:64 reacts and 1:128 does not, so the titer is 64.

    The titer is the reciprocal of the highest dilution that still reacts.

  5. Follow the algorithm: the specimen goes on for a treponemal test.

    Every reactive nontreponemal result needs treponemal confirmation.

Report the RPR as reactive with an endpoint titer of 1:64. The nonreactive undiluted reading was a prozone. The specimen goes on for treponemal testing.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A particle agglutination test for patient antibody is nonreactive undiluted but reactive at validated specimen dilutions. Which mechanism explains this pattern?

Incorrect. The antigen is a fixed reagent on the particles, so diluting the specimen lowers only patient antibody. A reaction limited by relative antigen excess would weaken further on dilution.

Correct. Excess patient antibody coats the particles and leaves too few antigen bridges. Dilution moves the reaction toward equivalence and restores visible agglutination.

Incorrect. Cross-reactivity is recognition of a related structure by the same binding site. It does not explain a reaction masked undiluted and restored as the reactants approach equivalence.

Hint
  1. Ask which reactant becomes more dilute when the specimen is diluted.
  2. The antigen is on the reagent particles in a fixed amount.

Review Lattice formation and reaction zones

Problem 2 of 3 · Hints available

In a valid agglutination-inhibition assay for a soluble hapten, the specimen is mixed with a limited amount of reagent antibody and then with hapten-coated particles. Which reading is the reactive outcome?

Incorrect. Agglutination means free reagent antibody remained to bridge the particles, so no hapten neutralized it. This is the nonreactive reading.

Incorrect. A reaction revealed by dilution describes a prozone in a direct antibody-detection test. An inhibition assay is read by whether agglutination is prevented.

Correct. Hapten in the specimen binds the limited reagent antibody, so none remains to bridge the coated particles. With valid controls, absent agglutination is the reactive outcome.

Hint
  1. Follow the limited reagent antibody. Ask where it goes when the specimen contains hapten.
  2. Ask what the coated particles need to clump.

Review Precipitation and agglutination

Problem 3 of 3 · On your own

A serum human chorionic gonadotropin (hCG) by sandwich immunoassay reads 950 mIU/mL, inside the measuring range. The requisition notes a suspected molar pregnancy, and the ordering physician expected a very high hCG. What do you do before release?

A hooked result can fall inside the measuring range. Releasing it without a dilution check can report a very high hCG as a modest one.

Accepted a low sandwich result without a dilution check

Extreme antigen excess saturates capture and detection antibodies separately, so a sandwich assay reads falsely low. Releasing an unexpectedly low value without dilution can report a very high concentration as low. Diluting and retesting gives the corrected, higher result.

A new specimen contains the same excess hCG and would saturate the antibodies again.

Heterophile antibodies usually bridge a sandwich and raise the result. This result is lower than expected.

A result far below the expected value in a sandwich assay can come from antigen excess. After dilution, a hooked result increases once corrected for the dilution factor.

Review Lattice formation and reaction zones

Use it

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

  • Cerebrospinal fluid (CSF) from Dario Labelle, 34, MRN 3308516, comes in for cryptococcal antigen testing.
  • The laboratory's method is latex agglutination with particles coated with anti-cryptococcal antibody. Its procedure titers every reactive specimen.
  • The positive control agglutinates, and the negative control stays smooth.
  • Undiluted CSF gives no agglutination.
  • The microbiology bench calls to say they see many encapsulated yeasts in the same CSF, so the specimen is retested at dilution. The 1:10 dilution agglutinates.
Decision 1 of 3

In this assay, what does agglutination mean?

That is the interpretation in a passive agglutination test, whose particles are coated with antigen. These particles are coated with antibody.

This is reverse passive agglutination. Antigen in the CSF bridges the antibody-coated particles into clumps.

An inhibition test has a limited amount of free reagent antibody and hapten-coated particles. Here the antibody is bound to the particles, and clumping shows antigen.

Review Precipitation and agglutination

Decision 2 of 3

Why did the undiluted CSF give no agglutination?

In this assay the CSF provides the antigen. The antibody is a fixed reagent on the particles, so the only excess the specimen can cause is antigen excess.

Both controls gave their expected reactions, so the run is valid.

So much antigen coats each particle's antibody that few bridges form between particles. Dilution reduces the excess, and clumping appears.

Review Lattice formation and reaction zones

Decision 3 of 3

What does the laboratory report?

The undiluted reading was lowered by antigen excess. The dilution shows the antigen is present at a high concentration.

The antigen is present. The procedure titers each reactive specimen, so the series is carried to its last reactive dilution, and that endpoint is reported.

A new CSF specimen would contain the same excess antigen. The dilution already showed the antigen on this specimen.

Review Lattice formation and reaction zones

The finding that matters is the reaction that appears only after dilution:

  • Undiluted CSF: no agglutination
  • 1:10 dilution: agglutination

In a test for antigen, a reaction that appears on dilution shows antigen excess. The report gives the titer from the full series.

For reference

Sources checked