Hello
Your father is having fever from 15 days with a gap of 4 days .
Serology test for dengue is negative ( indeterminate ) . The counts of platelets is not a parameter for dengue .
However, you didn't mention the other symptoms and type of fever , but may be due to these possibilities , these include:
1
Typhoid fever , diagnosis can be confirmed by Widal test , serology and
stool test .
2
Tuberculosis , is another most common cause of such fever (
long standing fever ,low grade ) . ESR is further an indication of tuberculosis or any other chronic infection.
3 Jaundice , this is most common cause beside above mentioned reasons for fever . Get in blood for HBASG for hepatitis .
However , there are so many other reasons of such fever , like UTI , bronchitis ,
sore throat .
But in my opinion consult a physician and get his opinion.
Good luck