During coding of a patient treated for both a malignant neoplasm and resultant dehydration, which diagnosis is recorded first?

When coding a patient who has both a malignant neoplasm and resultant dehydration, the malignant neoplasm should be recorded first because it is the principal diagnosis that necessitates the patient's treatment. In ICD-10-CM coding guidelines, the principal diagnosis is defined as the condition, after study, that is considered to be the primary reason for the patient's admission or treatment.

Since the malignant neoplasm is a serious condition that can lead to complications such as dehydration, it is essential to prioritize it in the coding process. The dehydration, while clinically significant, is a secondary condition that results from the underlying malignancy. Therefore, the proper coding sequence reflects the severity and primary nature of the malignant neoplasm in contributing to the patient’s overall clinical picture.

This ranking aligns with the coding guidelines that emphasize the importance of identifying the major reason for care when multiple conditions are present.