# AI instructs a nurse to read a tuberculin skin test (TST) at 24 hours instead of the required 48-72 hours, leading to false-negative interpretation

- **ID:** `medical/tuberculin-skin-test-reading-48h`
- **Domain:** medical
- **Category:** runtime_error
- **Error Code:** `TST_READING_24H`
- **Verification:** ai_generated
- **Fix Rate:** 92%

## Root Cause

TST induration peaks at 48-72 hours; reading at 24 hours misses delayed hypersensitivity reaction, causing false-negative rates up to 30%.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| CDC-TST-guidelines-2024 | active | — | — |
| WHO-TST-protocol-v3.2 | active | — | — |

## Workarounds

1. **Schedule the TST placement and reading appointments at the same time to ensure a 48-72 hour interval. Document both placement time and scheduled reading time in the patient record. If the patient cannot return at 48-72 hours, do not place the TST; use an IGRA blood test (e.g., QuantiFERON-TB Gold) instead.** (95% success)
   ```
   Schedule the TST placement and reading appointments at the same time to ensure a 48-72 hour interval. Document both placement time and scheduled reading time in the patient record. If the patient cannot return at 48-72 hours, do not place the TST; use an IGRA blood test (e.g., QuantiFERON-TB Gold) instead.
   ```
2. **If a TST was read at 24 hours and is negative, do not accept it as final. Instruct the patient to return at 48-72 hours for a proper reading. Document the early reading as 'preliminary – not valid for interpretation' and re-read at correct interval. If the patient cannot return, repeat the TST from scratch.** (90% success)
   ```
   If a TST was read at 24 hours and is negative, do not accept it as final. Instruct the patient to return at 48-72 hours for a proper reading. Document the early reading as 'preliminary – not valid for interpretation' and re-read at correct interval. If the patient cannot return, repeat the TST from scratch.
   ```
3. **Use a two-step TST protocol for serial testing: place a second TST 1-3 weeks after the first if initial test is negative. This boosts the immune response and reduces false negatives due to waned immunity.** (88% success)
   ```
   Use a two-step TST protocol for serial testing: place a second TST 1-3 weeks after the first if initial test is negative. This boosts the immune response and reduces false negatives due to waned immunity.
   ```

## Dead Ends

- **Assuming that any visible reaction at 24 hours is sufficient for interpretation** — Induration at 24 hours is often due to immediate hypersensitivity (Arthus reaction) or irritation, not a delayed-type hypersensitivity response to TB antigens. True induration develops later. (65% fail)
- **Believing that a negative reading at 24 hours can be confirmed by reading again at 48 hours** — If the test is read at 24 hours and recorded as negative, the nurse may not re-read at 48 hours, or the patient may not return. Even if re-read, the initial reading may bias the second assessment. (50% fail)
- **Using a digital caliper or ruler to measure induration at 24 hours and assuming it will be the same at 48 hours** — Induration size increases over time; a small or absent reaction at 24 hours may become positive at 48 hours. Measuring early gives a falsely low reading. (70% fail)