Infectious Physical Particle Ratio
Compare infectious and physical particle concentrations on compatible assays; supplied criterion is not a universal quality specification.
Description
Compare PFU/mL with a matched physical-particle concentration, calculate particles per PFU, and apply a supplied minimum ratio.
Plaque-forming units, or PFU, describe plaque-forming infectivity under an assay’s conditions. A physical-particle measurement counts material by another method and can include particles that do not form plaques. Reporting both quantities can distinguish infectivity from particle abundance.1
Dividing PFU/mL by physical particles/mL cancels the volume unit and yields an assay-based ratio. Its reciprocal gives physical particles per PFU. Both measurements must describe compatible samples and volume bases.
Infectious Physical Particle Ratio reports both forms and a supplied-criterion flag. Its model restricts PFU concentration to no greater than particle concentration and requires both values to be positive.
When to compare infectivity with particle abundance
Use the tool with independently measured concentrations from matched material. It helps report a ratio while preserving the measurement definitions and any criterion selected for the analysis.
- Express an infectivity-to-particle ratio as a fraction.
- Compare physical particles per PFU across matched assays.
How to enter concentrations and calculate the ratio
- Enter positive Plaque forming units per ml and Physical particles per ml in the same sample basis.
- Enter Minimum infectious fraction from 0 to 1.
- Run the tool and read Infectious fraction, Physical particles per pfu, and Meets supplied criterion.
- Keep the measurement methods with the ratio rather than treating it as an assay-independent particle property.
infectiousFraction = PFUperMl/particlesPerMl; physicalParticlesPerPfu = 1/infectiousFraction The criterion comparison is inclusive: infectiousFraction ≥ minimumInfectiousFraction. The tool rejects PFU greater than supplied physical particles instead of clipping the ratio.
{
"input": {
"plaqueFormingUnitsPerMl": 100,
"physicalParticlesPerMl": 10000,
"minimumInfectiousFraction": 0.005
},
"output": {
"infectiousFraction": 0.01,
"physicalParticlesPerPfu": 100,
"meetsSuppliedCriterion": true
}
}Equal concentrations give ratio 1 and one particle per PFU. A zero PFU input is rejected, so zero infectivity and an infinite reciprocal are not represented. Nonpositive particle counts, invalid thresholds, and nonfinite inputs also produce errors.
Limitations and interpretation
- The operational PFU ratio does not directly label each physical particle as infectious or noninfectious. Assay efficiency and aggregation affect interpretation.
- Genome-copy concentration is not automatically a physical-particle count, and TU or TCID50 should not be substituted for PFU without a justified measurement model.
- No uncertainty, assay recovery, sample matching, or release specification is assessed.
- The minimum fraction is supplied; a true flag establishes only that this comparison passed.
Alternative or Complementary methods
Calculate plaque titer from underlying plates when needed, and keep functional transduction-titer measurements in their own units. Independent particle measurements remain necessary for this ratio.
References
-
The Effects of Statistical Multiplicity of Infection on Virus Quantification and Infectivity Assays — Biophysical Journal (2018)
Similar or alternative tools
- Replicate Plaque Assay Titer
Calculate individual and pooled PFU/mL from plaque counts, volumes, and dilution fractions.
- Poisson Corrected Lentiviral Titer
Estimate functional TU/mL from transduced-cell fractions using a different assay endpoint.