Faint positive hpt w/ first morn urine, neg dr. Urine test later that day, blood test came back post. Why was drs urine test neg? Early testing?

Pregnancy tests. Blood tests are more sensitive than urine tests, especially in early pregnancy. First morning urine is usually more concentrated than urine produced later in the day. Eating and drinking can dilute the urine enough to cause a false-negative result. That is why the blood test is done for confirmation.
Early into pregnancy. Urine pregnancy test is recommend to have a first void urine because it is concentrated.Since your first void urine has shown faint positive, it is normal that later that day it would be negative because it is early pregnancy..It can takes 2 weeks after you missed your expected period to be positive in the urine.However, serum pregnancy test can detect positive as early as 1wk before period due.
Varying hormone type. False negative tests can be caused by limited or no detection of HCG isoforms. A tests inability to recognize specific isoforms could have resulted in your falsely negative results.