1. Why does the volume and color of urine produced by the kidneys vary so much? 2. Why does urine always contain urea? 3. Why are glucose and amino acids reabsorbed in the nephron? ( 4. Why can't the kidney just excrete the filtered plasma without adjusting its content? its Glucose: Normally negative. A positive result indicates the presence of more glucose in the blood being filtered than the kidney can reabsorb. Protein: Normally no protein is detected in urine because it's too large to be filtered at the glomerulus. Blood: A positive test indicates the presence of either red blood cells or hemoglobin. Either result is abnormal. The test may be positive even if no cells are seen under the microscope. Blood in urine is usually the result of some trauma to the renal tubule, ureter, bladder, or urethra. Ketone: Normally no ketone is present. Ketone in the blood comes from burning body fat for energy. Its presence in urine may indicate fasting, strenuous exercise, pregnancy, or diabetes. Bilirubin/urobilinogin: Bilirubin is normally negative. Bilirubin and urobilinogin are products formed from the breakdown of red blood cells and recycling of hemoglobin in the body. Their presence in the urine indicates an abnormality in the recycling of red blood cells. Blood cells may be dying too rapidly; there may be a reduction in liver function required to recycle normal numbers of red blood cells. Nitrite is a product of the bacterial reduction of nitrate and indicates the presence of a bacterial infection. Normally there is no nitrite in urine (normal urine is sterile). Painful urination is typical of lower urinary tract (urethra and bladder) infections. These infections usually travel up the urinary tract. Lower urinary tract infections can travel up into the kidney if left untreated. They may also be sexually transmitted. Antibiotics are used to treat infections. Bacterial culture (identification) is important to guide proper antibiotic therapy. Sediment: Common urine sediment includes epithelial cells, blood cells, casts and crystals. White blood cells suggest infection. Small numbers of red cells may be present but significant numbers are not normal. The presence of crystals indicates an increased risk of forming kidney stones. They can result from excess oxalate in the diet (nuts, dark green vegetables) and are made worse by conditions that cause calcium oxalate to precipitate out of solution (high concentration, high pH). Calcium oxalate stones are the most common type but uric acid and certain other minerals can precipitate out of urine.