Urinary Tract Infection (UTI): The presence of nitrites in the urine is a strong indicator of a UTI, as many gram-negative bacteria that cause UTIs can reduce nitrate to nitrite. Significant hematuria can also occur in the setting of a UTI, especially if the infection is severe or involves the upper urinary tract.

Kidney Stones: Hematuria is a common symptom of kidney stones, and while stones themselves do not produce nitrites, a secondary infection can. The combination of significant hematuria and positive nitrites could suggest a stone with an associated UTI.
Bladder Cancer: Although less common, bladder cancer can cause hematuria. While it does not directly produce nitrites, a tumor could potentially become infected, leading to a positive nitrite test.

Sepsis from a Urinary Source: In cases where a UTI leads to sepsis, prompt recognition and treatment are crucial. The presence of significant hematuria and positive nitrites, especially in the context of systemic symptoms like fever, tachycardia, or hypotension, should raise concern for sepsis.
Pyelonephritis: An upper urinary tract infection that can cause significant morbidity if not treated promptly. Symptoms might include flank pain, fever, and the urinalysis findings mentioned.

Schistosomiasis: A parasitic infection that can cause hematuria and, in some cases, might be associated with secondary bacterial infections leading to positive nitrites. This diagnosis would be more likely in individuals who have traveled to or reside in areas where the disease is endemic.
Tuberculosis of the Urinary Tract: Can cause hematuria and might be associated with a positive nitrite test if there is a secondary bacterial infection. This condition is rare in many parts of the world but should be considered in high-risk populations or those with exposure history.