Jess - you're totally right that failure to thrive is a symptom and should define, after workup, what the cause is. Now, there's some particular direction give in the Official Coding Guidelines regarding general conditions in the face of malignancy. If it's thought that the failure to thrive (better represented as severe malnutrition) is caused by the malignancy and nothing is done for the malignancy for this general symptom complex, and the patient is given nothing but general nutritional support, then the severe malnutrition should be the principal diagnosis. If the patient is brought in with cachexia defined as severe malnutrition due to the malignancy and NOTHING is done, then the malignancy is to be the principal diagnosis. Here's the references - tell me what you think:
Anemia associated with malignancy
When admission/encounter is for management of an anemia associated with the malignancy, and the treatment is only for anemia, the appropriate anemia code (such as code 285.22, Anemia in neoplastic disease) is designated as the principal diagnosis and is followed by the appropriate code(s) for the malignancy.
Management of dehydration due to the malignancy
When the admission/encounter is for management of dehydration due to the malignancy or the therapy, or a combination of both, and only the dehydration is being treated (intravenous rehydration), the dehydration is sequenced first, followed by the code(s) for the malignancy.
Symptoms, signs, and ill-defined conditions listed in Chapter 16 associated with neoplasms
Symptoms, signs, and ill-defined conditions listed in Chapter 16 characteristic of, or associated with, an existing primary or secondary site malignancy cannot be used to replace the malignancy as principal or first-listed diagnosis, regardless of the number of admissions or encounters for treatment and care of the neoplasm.
h. Admission/encounter for pain control/management
See Section 1.C.6.a.5 for information on coding admission/encounter for pain control/management
It's relatively easy - relatively!