What does it mean when they say that the urine has a lot of blood and white blood cells, but low for nitrates?

Nitrates = infection: However, low nitrates does not necessarily mean no infection. Most bacteria release nitrates in urine, some don't.Probably what you are told is that the presence of cells without nitrates in urine may suggest a kidney disease rather than urine infection. Still need to have urine culture which I am sure is done or pending to rule in or out infection.

UTI : White blood cells in urine suggest possible urinary tract infection. Red blood cells could be due to kidney stone, cancer, damage from infection, or even contamination from menstrual flow (in women). Leukocyte esterase and nitrites support diagnosis of uti, whether simple bladder infection (cystitis) or complicated kidney infection (pyelonephritis). Definitive answer is culture & sensitivity.