## **Core Concept**
Heparin is an anticoagulant commonly used in arterial blood gas (ABG) sampling to prevent clotting in the syringe. However, heparin can dilute the blood sample and affect the results of the ABG analysis.

## **Why the Correct Answer is Right**
Heparin interferes with the **pH** result of ABG because it is acidic in nature. When heparin is used in the syringe, it can dilute the blood sample and introduce an acidic substance, thereby lowering the pH reading. This effect is most significant on the pH measurement. Although it can also affect $pCO_2$ and $pO_2$, the impact on pH is more direct and critical.

## **Why Each Wrong Option is Incorrect**
- **Option A:** $pO_2$ can be affected by the dilutional effect of heparin but saying it is the primary interference might not be accurate.
- **Option B:** $pCO_2$ levels can also be influenced by heparin due to dilution, but similar to $pO_2$, it is not the most directly affected parameter.
- **Option D:** Bicarbonate ($HCO_3^-$) levels are calculated from pH and $pCO_2$ using the Henderson-Hasselbalch equation; thus, while pH affects bicarbonate levels, saying heparin directly interferes with bicarbonate might be indirect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when using a heparinized syringe for ABG sampling, it is crucial to ensure the syringe contains the minimum amount of heparin necessary to prevent clotting. Excess heparin can significantly affect ABG results, particularly the pH.

## **Correct Answer:** . pH