To diagnose GERD , in 24 hour ambulatory pH testing the percentage of the day that the pH was less than 4 is:
**Core Concept**
The diagnosis of gastroesophageal reflux disease (GERD) often involves assessing the frequency and duration of acid reflux episodes in the esophagus. Ambulatory 24-hour pH testing is a diagnostic tool used to quantify acid exposure in the esophagus.
**Why the Correct Answer is Right**
In ambulatory 24-hour pH testing, the esophageal pH is monitored continuously for 24 hours. The percentage of time the pH remains below 4 (acidic) is a key parameter used to diagnose GERD. This is because a pH of 4 or less is considered acidic and indicative of acid reflux. The DeMeester score, which combines the percentage of time pH is below 4 with other factors, is used to diagnose GERD. A higher percentage of time with pH below 4 indicates more severe acid reflux and a greater likelihood of GERD.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a specific percentage value, making it difficult to determine its accuracy.
* **Option B:** This option is incorrect because it does not accurately reflect the criteria used to diagnose GERD.
* **Option D:** This option is incorrect because it is either too high or too low, and does not accurately reflect the normal or pathological range for acid exposure in the esophagus.
**Clinical Pearl / High-Yield Fact**
The DeMeester score is a useful tool for diagnosing GERD, but it is not the only factor considered. A high percentage of time with pH below 4, combined with symptoms of GERD and failure of empiric therapy, is necessary for a diagnosis.
**Correct Answer: C. 4.2% - 6.5%**