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It's because of vitamin C.
Some dipsticks give lower or false-negative readings in the presence of high levels of ascorbic acid. If necessary, the test should be repeated at least24 hours after the last dose of vitamin C. Captopril (Capoten) may reduce the reagent pad’s sensitivity. Sensitivity is less in urines with high specific gravity, nitrites, or protein. In addition, specimens pre- served using formalin will yield a false-negative result. The dipstick is slightly more sensitive to free hemoglobin and myoglobin than to intact RBCs. If the urine sample is not mixed well before testing, a false-negative result can occur because the red cells tend to settle in the bottom of the container.
did you try to use strips from another company? and did you check the EXP date of the strips?
