This is a ready-to-use worksheet for computing moist heat lethality. It supports two tasks: computing the F0 delivered by a cycle from cold-spot temperature data, and sizing the required F0 from the sterilization approach. It is a calculation aid, not a validated tool; if you build it into a spreadsheet used for GxP decisions, validate that spreadsheet. Replace every <<FILL: ...>> placeholder with your own values. A worked filled specimen follows. Confirm every constant and threshold against your approved approach and the current standards.
Inputs
| Field | Entry |
|---|---|
| Sterilizer / cycle | <<FILL: ID>> |
| Reference temperature | 121.1 °C (250 °F), the F0 convention |
| Reference z-value | <<FILL: 10 °C for G. stearothermophilus moist heat>> |
| Sampling interval Δt | <<FILL: e.g. 1 min or 15 s>> |
| Cold-spot probe ID | <<FILL: the slowest-heating load location>> |
| BI organism and D₁₂₁ | <<FILL: e.g. G. stearothermophilus, D121 = 1.8 min from lot cert>> |
| Target SAL | <<FILL: 10^-6 for product contact>> |
Part A: the lethality formula
The instantaneous lethal rate at temperature T is:
L = 10^((T - 121.1) / z), with z = 10 °C.
F0 is the sum of L over the whole cycle, sampled at interval Δt:
F0 = Σ 10^((Tₜ - 121.1) / 10) × Δt
Lethality accrues during come-up and cool-down, not only during the hold, so read temperature across the entire cycle. Because the relationship is logarithmic, a few degrees above 121 °C contributes disproportionately.
Part B: F0 accumulation table (from cold-spot data)
Fill one row per sampling point. Sum the last column to get delivered F0.
| Time | T (°C) at cold spot | (T - 121.1) / 10 | L = 10^(…) | Contribution (L × Δt) |
|---|---|---|---|---|
<<FILL>> | <<FILL>> | <<FILL>> | <<FILL>> | <<FILL>> |
<<FILL>> | <<FILL>> | <<FILL>> | <<FILL>> | <<FILL>> |
| … | ||||
| Delivered F0 (sum) | <<FILL>> |
Part C: overkill check
| Field | Entry |
|---|---|
| Delivered F0 at cold spot | <<FILL: from Part B>> |
| Overkill target | ≥ 12 min |
| Log reductions delivered = exposure time / D | <<FILL: e.g. 30 min / 1.8 = 16.7 logs>> |
| Meets 12-log overkill? | <<FILL: Yes / No>> |
Part D: bioburden design (sizing the required F0)
Use when the product cannot take a full overkill cycle.
| Step | Formula | Entry |
|---|---|---|
| Maximum pre-sterilization bioburden N₀ | measured, count per unit | <<FILL>> |
| Most resistant organism D₁₂₁ | measured or referenced | <<FILL>> |
| Log reductions to reach SAL | log₁₀(N₀) - log₁₀(SAL) | <<FILL: e.g. 2 - (-6) = 8>> |
| Required F0 at cold spot | log reductions × D₁₂₁ | <<FILL: e.g. 8 × 0.5 = 4.0 min>> |
| Design margin added | justified | <<FILL>> |
| Design F0 target | required + margin | <<FILL>> |
The bioburden design only holds if the count and resistance stay within the assumed limits; a rise above them means the delivered F0 no longer reaches the target SAL.
Part E: physical vs biological F0
| Field | Entry |
|---|---|
| Physical F0 (from Part B) | <<FILL>> |
| Biological F0 (from BI log reduction) | <<FILL>> |
| Consistent? | <<FILL: Yes / No; investigate if not>> |
References
ISO 11139 (definitions of D-value, z-value, SAL, F0); ISO 17665 (moist heat validation). USP <1229.2> (moist heat sterilization of aqueous liquids); USP <1211> (sterility assurance). 21 CFR 211.113(b); EU GMP Annex 1 (2022).
Describe copyrighted standards in your own words; confirm current editions before use.
Filled specimen
Part B accumulation for a probe reading, at one-minute intervals: 112, 116, 119, 121, 122, 121, 121, 121, 118, 114 °C.
| Time (min) | T (°C) | (T - 121.1)/10 | L | Contribution |
|---|---|---|---|---|
| 1 | 112 | -0.91 | 0.123 | 0.123 |
| 2 | 116 | -0.51 | 0.309 | 0.309 |
| 3 | 119 | -0.21 | 0.617 | 0.617 |
| 4 | 121 | -0.01 | 0.977 | 0.977 |
| 5 | 122 | 0.09 | 1.230 | 1.230 |
| 6 | 121 | -0.01 | 0.977 | 0.977 |
| 7 | 121 | -0.01 | 0.977 | 0.977 |
| 8 | 121 | -0.01 | 0.977 | 0.977 |
| 9 | 118 | -0.31 | 0.490 | 0.490 |
| 10 | 114 | -0.71 | 0.195 | 0.195 |
| Delivered F0 | ≈ 6.87 min |
Two lessons every practitioner should read off this table: lethality accrued in the ramp and cool-down (rows 1 to 3 and 9 to 10 add more than 1.7 min between them, not zero), and the single 122 °C point (row 5) contributed 23 percent more than a 121.1 °C point would.
Part D bioburden design. N₀ = 100 (10²); most resistant organism D₁₂₁ = 0.5 min; target SAL 10⁻⁶. Log reductions = 2 - (-6) = 8. Required F0 = 8 × 0.5 = 4.0 min. With a justified 50 percent margin, design target ≈ 6 min, well below the ≥ 12 min an overkill cycle would demand, which is why the bioburden route is chosen for a heat-labile product.
Common mistakes this worksheet prevents
- Computing F0 only over the hold, understating delivered lethality.
- Taking temperature from the chamber or drain rather than the load cold spot.
- Sizing a bioburden cycle without a controlled, monitored bioburden limit behind the assumed N₀ and D-value.
- Releasing at the bare calculated minimum with no design margin.
How to adapt this worksheet
- Set Δt to your data logger’s interval; shorter intervals give a more accurate integral, especially through fast ramps.
- Use the D-value from the actual BI lot certificate, not a generic value.
- If you implement this as a spreadsheet used for GxP release, validate it (see infrastructure qualification and spreadsheet validation).
- Keep the physical-versus-biological F0 comparison in Part E; agreement is the evidence that the cycle behaves as the physics predicts.