Calculator
Formula Used
Earliest read time =
Placement date and time + 48 hours.
Latest usual read time =
Placement date and time + 72 hours.
Days to read =
Reading hours ÷ 24.
Elapsed hours at return =
Return date and time - Placement date and time.
The default reading window is 48 to 72 hours. You may edit it for local policy. A trained reader should measure induration in millimeters.
How to Use This Calculator
- Enter the date and time when the TB skin test was placed.
- Keep 48 hours as the default earliest reading time.
- Keep 72 hours as the default latest usual reading time.
- Add the planned return date and time, if known.
- Enter induration in millimeters only when a trained reader measures it.
- Select a threshold group only when a clinician provides the proper context.
- Press the calculate button to view the reading window.
- Use CSV or PDF export for appointment records.
Example Data Table
| Placement | Earliest Read | Latest Usual Read | Return Time | Status |
|---|---|---|---|---|
| Monday 9:00 AM | Wednesday 9:00 AM | Thursday 9:00 AM | Wednesday 10:30 AM | Within valid window |
| Tuesday 2:00 PM | Thursday 2:00 PM | Friday 2:00 PM | Thursday 10:00 AM | Too early |
| Friday 8:00 AM | Sunday 8:00 AM | Monday 8:00 AM | Monday 3:00 PM | Past usual window |
TB Skin Test Reading Window Guide
A TB skin test is time sensitive. The test is usually placed under the skin of the forearm. The reading is not done right away. The immune response needs time to appear. This calculator helps you plan that reading visit with clear dates.
Why Timing Matters
The usual reading window starts at 48 hours. It ends at 72 hours. Reading before the window may miss the full reaction. Reading after the window may reduce accuracy. This is why many clinics give a strict return appointment. The patient must return on time.
What the Tool Calculates
The calculator adds hours to the placement date and time. It then shows the earliest reading time. It also shows the latest usual reading time. A preferred time can be added too. Many clinics prefer a time close to 72 hours. That choice may depend on clinic policy.
Return Time Status
The optional return date helps compare a planned visit. If the visit is before 48 hours, the tool marks it too early. If the visit is between 48 and 72 hours, it marks the timing as valid. If the visit is after 72 hours, the tool warns that the usual window has passed.
Induration Measurement
The TB skin test is read by measuring induration. Induration means firm swelling. Redness alone is not the measurement. The reader records the width in millimeters. The result depends on the measurement and risk category. This tool can compare a measurement with a selected threshold. It does not replace medical judgment.
Clinic Planning
The calculator includes clinic opening and closing times. These fields help users think about real appointment hours. A reading window may fall on a weekend or holiday. In that case, call the clinic early. Do not wait until the final hour.
Record Keeping
Many employers, schools, and clinical programs need TB screening records. The CSV button saves a simple table. The PDF button saves a readable summary. Keep these files with your health paperwork. Bring official clinic documents when required.
Safe Use
This calculator is for scheduling support. It cannot confirm infection. It cannot decide treatment. It cannot replace a trained health worker. Use it to avoid missed reading windows. Use professional care for final reading and interpretation.
Frequently Asked Questions
1. When should a TB skin test be read?
It is usually read between 48 and 72 hours after placement. The calculator uses that window by default.
2. Can the test be read before 48 hours?
Early reading may not show the full reaction. The tool marks early return times as too early.
3. What happens after 72 hours?
The usual reading window has passed. Contact the clinic. A repeat test may be required by policy.
4. Does this calculator diagnose TB?
No. It only calculates timing and basic threshold notes. A trained health worker must read and interpret results.
5. What is induration?
Induration is firm swelling at the test site. It is measured in millimeters across the forearm.
6. Should redness be measured?
No. Redness alone is not the standard measurement. The firm raised area is measured instead.
7. What does the 5 mm threshold mean?
It is used for selected higher risk groups. A clinician should decide whether that threshold applies.
8. What does the 10 mm threshold mean?
It is used for some moderate risk groups. Local guidance and clinical history should be considered.
9. What does the 15 mm threshold mean?
It may apply to people with no known TB risk factors. A clinic should confirm the category.
10. Can I use this for workplace screening?
Yes, for timing records. Always follow workplace, clinic, and public health requirements for official documentation.
11. Can I change the reading hours?
Yes. The form allows custom minimum, maximum, and preferred hours for local rules or special instructions.
12. Why add a preferred reading hour?
It helps plan a practical appointment inside the allowed window. Some clinics prefer a specific reading time.
13. Can I export my result?
Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple printable summary.
14. What should I do if I miss the window?
Call the clinic as soon as possible. They can decide whether you need another test or a different screening option.