PRNT Dilution Endpoint

Find the highest observed dilution meeting a supplied plaque-reduction criterion, preserving out-of-range censoring.

Description

Find the highest tested reciprocal dilution meeting a plaque-reduction criterion and report whether the endpoint remains beyond the tested range.

A plaque-reduction neutralization test, or PRNT, compares plaque counts in the presence of a test specimen with a control count. Reduction is expressed relative to the control. A reciprocal-dilution endpoint summarizes the highest tested specimen dilution that still meets a chosen reduction criterion.^[1]

For a control count of 100 plaques, a specimen count of 20 means reduction 0.8, or 80%. A reciprocal dilution of 40 represents a 1:40 dilution. If the criterion is still met at the highest tested dilution, the observed result is a lower bound on an endpoint outside the tested range.

PRNT Dilution Endpoint returns reduction fractions, the highest qualifying tested reciprocal, a qualifying-result flag, and a right-censoring flag. It does not interpolate a continuous endpoint between dilutions.

When to read an observed plaque-reduction endpoint

Use the tool for a scored dilution series with increasing reciprocal dilutions and nondecreasing plaque counts. Select the reduction criterion outside the calculator and retain the tested range with the endpoint.

Identify the last observed dilution meeting a supplied PRNT threshold.
Keep above-range endpoints distinct from fully bracketed results.
Distinguish a nonqualifying series from invalid assay input.

How to enter the PRNT series and select its endpoint

Enter positive Reciprocal dilutions in strictly increasing order and aligned nonnegative integer Plaque counts.
Enter a positive integer Control plaques count.
Enter Minimum reduction fraction from 0 to 1, such as 0.8 for 80%.
Run the tool. Read Endpoint reciprocal together with Endpoint observed and Right censored.

reduction_i = 1 − plaqueCounts_i/controlPlaques; endpoint = largest tested reciprocal whose reduction_i ≥ minimumReductionFraction

Equality qualifies. The implementation requires plaque counts not to decrease as reciprocal dilution increases.

Series result	Outputs
At least one qualifies and the final dilution fails	Highest qualifying reciprocal; endpointObserved true; rightCensored false.
The final dilution qualifies	Final reciprocal; endpointObserved true; rightCensored true.
No dilution qualifies	Reciprocal 0 sentinel; endpointObserved false; rightCensored false.

json

{
  "input": {
    "reciprocalDilutions": [
      10,
      20,
      40,
      80
    ],
    "plaqueCounts": [
      5,
      10,
      30,
      60
    ],
    "controlPlaques": 100,
    "minimumReductionFraction": 0.8
  },
  "output": {
    "reductionFractions": [
      0.95,
      0.9,
      0.7,
      0.4
    ],
    "endpointReciprocal": 20,
    "endpointObserved": true,
    "rightCensored": false
  }
}

Plaque counts above the control are accepted and give negative reduction fractions rather than clipped zero values. Empty/misaligned lists, duplicate dilution values, decreasing plaque counts, or invalid numeric domains produce errors.

Limitations of an observed PRNT endpoint

Endpoint observed means that at least one tested dilution qualifies. It does not mean the true endpoint has been located when rightCensored is true.
No interpolation, fitted neutralization curve, replicate averaging, confidence interval, or repair of noisy nonmonotone counts is supplied.
The criterion and control count are caller-supplied, with no built-in assay protocol or protection threshold.
A zero reciprocal output is a sentinel for no qualifying dilution, not a measured zero antibody titer.

Alternative or Complementary dilution and antibody summaries

Preserve control quality, assay conditions, and censoring notation with the result. A TCID50 estimate measures infectious titer from well responses, while a paired HAI rise measures a ratio of supplied antibody titers. They address different assay endpoints.

References

Evaluating Humoral Immunity against SARS-CoV-2: Validation of a Plaque-Reduction Neutralization Test and a Multilaboratory Comparison of Conventional and Surrogate Neutralization Assays — Microbiology Spectrum (2021)

Similar or alternative tools

Spearman Karber Infectious Titer
Estimate TCID50/mL from positive-well fractions across a regular dilution series.
Paired HAI Titer Rise
Compare acute and convalescent reciprocal HAI titers with a supplied fold-rise criterion.