Calculator
Example Data Table
| Pattern | Start Level | Hours | Half Life | Cutoff | CV | Use |
|---|---|---|---|---|---|---|
| Single estimate | 120 | 96 | 60 | 50 | 25% | Basic screen model |
| Weekly pattern | 150 | 168 | 72 | 50 | 30% | Pattern comparison |
| Frequent pattern | 240 | 240 | 96 | 15 | 35% | Confirmation comparison |
| Custom lab rule | 80 | 120 | 48 | 20 | 20% | Custom cutoff test |
Formula Used
Body fat factor: 1 + ((body fat percent - 20) × 0.015)
Adjusted start: starting level × frequency factor × body fat factor × specimen factor
Decay estimate: adjusted start × 0.5(hours since last exposure ÷ half life)
Standard deviation: estimated current level × measurement variation percent
Z score: (cutoff - estimated current level) ÷ standard deviation
Chance above cutoff: 1 - normal cumulative distribution of z score
Observed positive chance: chance above cutoff × sensitivity + chance below cutoff × false positive rate
The formula is educational. Real laboratory results can differ because testing methods, biology, and policies vary.
How to Use This Calculator
Choose a test type or enter a custom cutoff. Add the estimated starting level, elapsed hours, and half life. Select the use pattern modifier. Enter body fat, specimen variation, measurement variation, sensitivity, and specificity. Press Calculate. Review the result above the form. Export the result with CSV or PDF.
Statistical THC Screening Estimate
What This Calculator Does
THC screening is not a simple countdown. The body stores and clears cannabinoids unevenly. A single number cannot cover every person. This calculator treats the question as a statistical estimate. It starts with an assumed starting concentration. Then it applies half life decay. The result is compared with a selected cutoff.
Why the Model Uses Decay
The model uses an exponential decay curve. That curve is common in medicine, chemistry, and statistics. It means a value falls by half during each half life. A longer half life keeps the estimated level higher. A shorter half life lowers it faster. The calculator also adjusts for use pattern and body fat. These fields are rough modifiers. They should not be treated as proof.
How Uncertainty Is Handled
Drug screens also have measurement uncertainty. A lab result can vary around the real value. This tool uses a coefficient of variation. It creates a standard deviation around the estimate. The z score then estimates the chance of crossing the cutoff. Sensitivity and specificity are added. They show how test performance can affect an observed result.
Reading the Output
The output includes an estimated current level. It also shows a confidence range. The range helps explain uncertainty. The probability is not a promise. It is a model based on your entries. Real screening can differ because biology differs. Test type, method, collection quality, metabolism, and lab rules matter.
Responsible Use
Use this page for education and planning only. Do not use it to bypass workplace, court, school, or medical requirements. It cannot validate abstinence. It cannot confirm impairment. It also cannot replace professional advice. For official decisions, rely on certified testing and qualified experts. Enter conservative values when unknown. Review the formula section before using results. Export the table only for your own records.
Assumptions and Limits
A statistics page should make assumptions visible. This is why each field has a clear label. You can change cutoff, half life, and variation. You can also compare patterns without changing the code. The example table gives sample inputs. It only shows how the calculator behaves.
Probability Reminder
Interpreting the result needs care. A low estimate can still produce a positive result. A high estimate can still produce a negative result. Rare outcomes happen in probability models. Documentation, policies, and expert review remain important for real cases.
FAQs
1. Can this calculator guarantee a drug test result?
No. It gives a statistical estimate only. Real results depend on biology, lab method, cutoff, collection rules, and testing quality.
2. What does the cutoff mean?
The cutoff is the decision level used by a test. Results above it may be treated as positive under that test rule.
3. Why does half life matter?
Half life controls the decay curve. A longer half life means the estimated level falls more slowly over time.
4. What is measurement variation?
Measurement variation describes possible spread around the estimate. The calculator uses it to build probability and confidence range values.
5. What do sensitivity and specificity mean?
Sensitivity estimates how often a test detects true positives. Specificity estimates how often it correctly reports true negatives.
6. Should I use this for legal decisions?
No. Legal, workplace, court, and medical decisions should rely on official testing, written policies, and qualified professional advice.
7. Why is the result a probability?
Testing has uncertainty. Biology also varies. A probability communicates risk better than a single fixed prediction.
8. Can I export my calculation?
Yes. Use the CSV or PDF buttons. They export the current result after calculation, or example data before calculation.