A female patient with atrial fibrillation has the following lab results: Hemoglobin of 11 g/dl, a platelet count of 150,000, an INR of 2.5, and potassium of 2.7 mEq/L. Which result is critical and should be reported to the physician immediately?

The critical lab result that should be reported to the physician immediately in this case is the potassium level of 2.7 mEq/L. A potassium imbalance, especially in a patient with a history of dysrhythmia like atrial fibrillation, can be life-threatening and lead to cardiac distress. Low potassium levels (hypokalemia) can predispose the patient to dangerous arrhythmias, including worsening atrial fibrillation. Hemoglobin of 11 g/dl, platelet count of 150,000, and an INR of 2.5 are within acceptable ranges and not as immediately concerning as a low potassium level in this clinical context.