Replicate Plaque Assay Titer

Estimate infectious plaque titer from effective-volume-weighted replicate plates, retaining individual plate estimates.

Description

Calculate individual and pooled PFU-per-milliliter estimates from plaque counts, weighting replicate plates by effective original-sample volume.

A plaque assay measures infection events that produce countable plaques under the assay conditions. A plaque-forming unit, or PFU, is an operational infectious unit; it is distinct from a physical-particle or genome-copy count. Plaque counts depend on the inoculated volume and the fraction of original sample present after dilution.^[1]

Each plate therefore represents an effective original-sample volume equal to inoculum volume times dilution fraction. Pooling all plaques over the sum of those effective volumes yields a volume-weighted titer. It differs from a simple average of per-plate titers when the effective volumes differ.

Replicate Plaque Assay Titer returns the pooled PFU/mL estimate, each plate’s estimate, and total plaques. It retains plate-to-plate variation for inspection rather than hiding it in a single value.

When to combine replicate plaque measurements

Use this tool for selected countable plates representing the same original sample. Supply each plate’s own inoculum volume and dilution, and review whether the assay readout supports treating the counts as comparable.

Calculate per-plate titers from different dilutions or volumes.
Pool compatible replicates using their original-sample exposures.
Retain zero plaque counts as observations rather than silently dropping them.

How to enter replicate plates and calculate weighted titer

Enter a nonempty Plaque counts list of nonnegative integers.
Enter equally long Inoculum volumes ml and Dilution fractions lists in matching plate order.
Use positive volumes and dilution fractions greater than zero and at most one; a 1:100 dilution is 0.01.
Run the tool and read Plate pfu per ml alongside Pooled pfu per ml and Total plaques.

v_i = inoculumVolumesMl_i × dilutionFractions_i; platePfuPerMl_i = plaqueCounts_i/v_i; pooledPfuPerMl = sum(plaqueCounts_i)/sum(v_i)

The implementation sums counts and effective volume separately; it does not average plate estimates with equal weight.

json

{
  "input": {
    "plaqueCounts": [
      10,
      20
    ],
    "inoculumVolumesMl": [
      0.1,
      0.1
    ],
    "dilutionFractions": [
      0.01,
      0.01
    ]
  },
  "output": {
    "pooledPfuPerMl": 15000,
    "platePfuPerMl": [
      10000,
      20000
    ],
    "totalPlaques": 30
  }
}

json

{
  "input": {
    "plaqueCounts": [
      10,
      30
    ],
    "inoculumVolumesMl": [
      0.1,
      0.1
    ],
    "dilutionFractions": [
      0.01,
      0.02
    ]
  },
  "output": {
    "pooledPfuPerMl": 13333.333333333334,
    "platePfuPerMl": [
      10000,
      15000
    ],
    "totalPlaques": 40
  }
}

All-zero counts produce zero titers. Empty or misaligned lists, fractional counts, nonpositive volumes, and invalid dilution fractions produce errors.

Limitations of pooled plaque arithmetic

Plate selection, countable range, overlapping plaques, assay efficiency, and identification of plaques are not evaluated.
Replicates must describe compatible exposures to the same original sample. Pooling cannot correct a systematic assay difference.
No confidence interval, replicate outlier exclusion, or detection-limit calculation is provided. Zero observed plaques do not prove absence of infectious material.
PFU is an assay endpoint and is not automatically interchangeable with TCID50, transducing units, or physical particle counts.

Alternative or Complementary infectious-titer summaries

Use endpoint-dilution calculations for binary well responses and preserve their TCID50 units. When a physical-particle measurement is available independently, an infectious-to-physical ratio can summarize another property of the sample without converting one assay into another.

References

The Effects of Statistical Multiplicity of Infection on Virus Quantification and Infectivity Assays — Biophysical Journal (2018)

Similar or alternative tools

Spearman Karber Infectious Titer
Calculate TCID50/mL from a complete regular dilution series of positive and tested wells.
Infectious Physical Particle Ratio
Compare independently supplied PFU/mL and physical-particle concentration.