According to the ICD-10-CM Official Guidelines for Coding and Reporting FY 2023, the principal diagnosis is defined as "the condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care"2. In this case, the patient was admitted with shortness of breath, productive yellow sputum, and a temperature of 101.2, which are signs and symptoms of pneumonia. The CXR confirmed the diagnosis of LLL pneumonia, which is a serious condition that requires inpatient care. Therefore, LLL pneumonia is the principal diagnosis.

The secondary diagnoses are defined as "all conditions that coexist at the time of admission, that develop subsequently, or that affect the treatment received and/or the length of stay"2. In this case, the patient had bilateral pleural effusion and kidney failure at the time of admission, which are coexisting conditions that affect the treatment received and/or the length of stay. The renal consult documented AKI with ATN, which is a more specific diagnosis than kidney failure and reflects the severity and etiology of the condition. Therefore, AKI with ATN and bilateral pleural effusion are secondary diagnoses.

References:

CDI Week 2020 Q&A: CDI and key performance indicators1

ICD-10-CM Official Guidelines for Coding and Reporting FY 20232