**Core Concept**
Gettler's test is a clinical laboratory method used to determine the chloride levels in the serum and the chloride levels in the blood of the right and left sides of the heart. This test is primarily employed in the diagnosis of potassium chloride poisoning, specifically in cases of digitalis poisoning, where it can help differentiate between the two sides of the heart.

**Why the Correct Answer is Right**
Gettler's test detects the chloride levels in the blood of the right and left sides of the heart, which helps in diagnosing potassium chloride poisoning. In cases of digitalis poisoning, the test can help differentiate between the two sides of the heart by analyzing the chloride levels. The test is based on the principle that potassium chloride poisoning results in a higher chloride level in the left side of the heart compared to the right side.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Gettler's test is not used to detect chloride levels in the serum only, but rather in the blood of the right and left sides of the heart.

**Option B:** This option is incorrect because Gettler's test is not used to detect chloride levels in the blood of the right side of the heart only, but rather in the blood of both the right and left sides of the heart.

**Option C:** This option is incorrect because Gettler's test is not used to detect sodium levels in the blood, but rather chloride levels.

**Clinical Pearl / High-Yield Fact**
Gettler's test is an important diagnostic tool in cases of digitalis poisoning, where it can help differentiate between the two sides of the heart by analyzing the chloride levels. This test is a classic exam trap, and students should remember that it is used to detect chloride levels in the blood of the right and left sides of the heart.

**Correct Answer: D. Chloride levels in the serum and the blood of the right and left sides of the heart.**