Calculator
Example Data Table
| Test type | Initial level | Cutoff | Hours | Half life | Expected result |
|---|---|---|---|---|---|
| Urine screen | 150 | 50 | 72 | 36 | Moderate estimate |
| Saliva screen | 45 | 25 | 24 | 18 | Uncertain estimate |
| Lab confirmation | 80 | 15 | 96 | 40 | Needs review |
Formula Used
Effective half life: effective half life = half life × frequency factor ÷ metabolism factor.
Expected concentration: expected level = starting level × 0.5time ÷ effective half life.
Combined uncertainty: combined SD = √(standard deviation² + lab margin SD²).
Z score: z = (cutoff − expected level) ÷ combined SD.
Model pass probability: probability = normal cumulative distribution of z.
Final probability: final estimate = 75% model probability + 25% prior pass rate.
How to Use This Calculator
- Select the test type and substance group.
- Enter the estimated starting level and cutoff.
- Add the hours since last exposure.
- Enter a half life estimate and standard deviation.
- Use conservative factors when details are unknown.
- Press Calculate to view the result above the form.
- Use CSV or PDF buttons to save the calculation.
Statistical View of Test Risk
What the Estimate Means
A drug test result is not decided by one simple number. It depends on the test method, the cutoff, the time since last exposure, and normal biological variation. This calculator treats the question as a probability problem. It does not promise a pass. It does not replace professional advice. It gives a clear estimate from the inputs you provide.
Why Uncertainty Matters
Testing has uncertainty. Labs use thresholds. Screens can vary. Bodies also clear substances at different rates. A person may enter the same values twice and still face a range of possible outcomes. That is why the calculator uses standard deviation and confidence level. These fields show how much the estimate may move around the central prediction.
How the Model Works
The model starts with an estimated starting concentration. It then applies half life decay. Each half life cuts the expected concentration in half. A metabolism factor can shorten or lengthen the effective half life. A use frequency factor can also stretch the estimate. The final concentration is compared with the chosen cutoff. The gap is converted into a z score.
Reading the Result
A high pass probability means the estimated concentration is likely below the cutoff. A low probability means the estimated level may stay above it. Middle values are uncertain. They deserve caution. The result panel also shows a concentration range. Use that range to understand best case and worst case movement.
Responsible Use
This tool should support honest planning. It should not be used to cheat, dilute, substitute, or avoid a required test. Results can affect jobs, licenses, treatment plans, and legal matters. Always follow the rules that apply to your test. Speak with a qualified professional when the outcome is important. Good inputs create better estimates, but no calculator can remove real testing uncertainty.
Advanced Inputs
The advanced fields are useful when you have reliable data. Use conservative values when you are unsure. A larger standard deviation widens the range. A higher cutoff usually improves the pass estimate. A longer time since last exposure lowers the predicted concentration. The prior pass rate adds background judgment. It should not overpower measured inputs. Keep notes for future comparisons.
FAQs
Can this calculator guarantee I will pass?
No. It only gives a statistical estimate from your inputs. Real tests can differ because of sample handling, lab methods, biology, and cutoff rules.
What is the most important input?
The cutoff, expected starting level, time since last exposure, and half life usually matter most. Weak input data creates a weaker estimate.
What does standard deviation mean here?
Standard deviation represents uncertainty around the expected concentration. A larger value makes the result range wider and less certain.
Why does the test type matter?
Different test types can use different cutoffs, sample sources, and review methods. Enter the cutoff supplied by the testing program when available.
Should I use this for legal decisions?
No. Legal, workplace, medical, or compliance decisions need qualified advice. This calculator is only a general statistics tool.
Does hydration change this calculation?
This tool does not advise dilution or sample manipulation. Follow all test rules. Use the lab variation field only for measurement uncertainty.
What if I do not know the half life?
Use a conservative estimate and review the probability range. Unknown half life values can strongly affect the final result.
Why is there a prior pass rate?
The prior pass rate adds background judgment to the model. Use 50 percent when you have no reliable prior information.