Poisson Corrected Lentiviral Titer

Estimate functional lentiviral titer with Poisson multi-hit correction and explicit assay volume and dilution.

Description

Estimate mean multiplicity and functional transducing units per milliliter from a transduced-cell fraction, cell count, inoculum volume, and dilution.

Functional transduction assays measure the fraction of target cells with the defined positive readout. A positive cell can have received more than one effective transducing event, so simply counting positive cells can underestimate the mean event count. A Poisson model relates the zero-event fraction to mean multiplicity of infection, or MOI.^[1]

Under that model, a transduced fraction f corresponds to mean multiplicity −ln(1−f). Multiplying by the target-cell count estimates transducing events in the inoculum. Dividing by inoculum volume and its fraction of original stock gives functional transducing units per milliliter, abbreviated TU/mL.

Poisson Corrected Lentiviral Titer performs this correction with explicit volume and dilution. TU/mL describes the supplied functional assay; it is not a physical-particle concentration or an automatically interchangeable PFU titer.

When to estimate a functional titer from transduction fractions

Use the calculator after an assay has provided the transduced fraction and the number of exposed target cells. Confirm that the readout and assay conditions support the supplied Poisson interpretation.

Correct a positive-cell fraction for possible multiple events per cell.
Convert an assay at a known dilution back to original-stock TU/mL.
Compare matched functional assay calculations with recorded cell counts and volumes.

How to enter assay quantities and calculate Poisson-corrected titer

Enter Transduced fraction from 0 up to, but excluding, 1.
Enter Target cells as a positive integer count of cells exposed in the assay.
Enter positive Inoculum volume ml and Dilution fraction greater than 0 and no greater than 1; for a 1:100 dilution, enter 0.01.
Run the tool and read Estimated moi and Transducing units per ml.

estimatedMoi = −ln(1−f); transducingUnitsPerMl = estimatedMoi×targetCells/(inoculumVolumeMl×dilutionFraction)

The implementation evaluates −log1p(−f), which avoids loss of precision from first subtracting very small fractions from one.

json

{
  "input": {
    "transducedFraction": 0.5,
    "targetCells": 1000,
    "inoculumVolumeMl": 0.1,
    "dilutionFraction": 0.01
  },
  "output": {
    "estimatedMoi": 0.6931471805599453,
    "transducingUnitsPerMl": 693147.1805599453
  }
}

A zero transduced fraction gives zero estimated multiplicity and titer. Fraction 1 is rejected because the inverse is unbounded. Invalid fractions, noninteger cell counts, and nonpositive volumes produce input errors.

Limitations of a Poisson functional-titer estimate

The model assumes independent effective events and an appropriate susceptible target population. Heterogeneity, aggregation, and readout saturation can change the interpretation.
The tool does not score cells, subtract background positivity, establish assay quality, or calculate uncertainty.
Results are sensitive to fraction estimates near one. A complete positive readout does not establish a finite titer here.
Cell type, assay timing, transduction definition, and conditions must accompany TU/mL comparisons. PFU and particle counts measure other quantities.

Alternative or Complementary multiplicity and infectious assays

Use a target-fraction inversion when only the Poisson multiplicity is needed. A plaque assay calculates PFU/mL from a different functional endpoint and should retain its own assay units.

References

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

Similar or alternative tools

Target Transduction MOI
Invert a target transduced fraction to its Poisson mean multiplicity without a stock-titer conversion.
Replicate Plaque Assay Titer
Estimate PFU/mL from plaque counts, volumes, and dilution fractions in a plaque assay.