Chronic Kidney Disease Diagnosis
Chronic kidney disease usually causes no symptoms in its early stages. Only lab tests can detect any developing problems. Anyone at increased risk for chronic kidney disease should be routinely tested for development of this disease.
- Urine, blood, and imaging tests (X-rays) are used to detect kidney disease, as well as to follow its progress.
- All of these tests have limitations. They are often used together to develop a picture of the nature and extent of the kidney disease.
- In general, this testing can be performed on an outpatient basis.
Urinalysis: Analysis of the urine affords enormous insight into the function of the kidneys. The first step in urinalysis is doing a dipstick test. The dipstick has reagents that check the urine for the presence of various normal and abnormal constituents including protein. Then, the urine is examined under a microscope to look for red and white blood cells, and the presence of casts and crystals (solids).
Only minimal quantities of albumin (protein) are present in urine normally. A positive result on a dipstick test for protein is abnormal. More sensitive than a dipstick test for protein is a laboratory estimation of the urine albumin (protein) and creatinine in the urine. The ratio of albumin (protein) and creatinine in the urine provides a good estimate of albumin (protein) excretion per day.
Twenty-four hour urine tests: This test requires the patient to collect all of their urine for 24 consecutive hours. The urine may be analyzed for protein and waste products (urea nitrogen, and creatinine). The presence of protein in the urine indicates kidney damage. The amount of creatinine and urea excreted in the urine can be used to calculate the level of kidney function and the glomerular filtration rate (GFR).
Glomerular filtration rate (GFR): The GFR is a standard means of expressing overall kidney function. As kidney disease progresses, GFR falls. The normal GFR is about 100 to 140 mL/min in men and 85 to 115 mL/min in women. It decreases in most people with age. The GFR may be calculated from the amount of waste products in the 24-hour urine or by using special markers administered intravenously. An estimation of the GFR (eGFR) can be calculated from the patient's routine blood tests. It is not as accurate in patients younger than 18, pregnant patients, and those who are very muscular or who are very overweight. Patients are divided into five stages of chronic kidney disease based on their GFR (see Table 1 above).
Creatinine and urea (BUN) in the blood: Blood urea nitrogen and serum creatinine are the most commonly used blood tests to screen for and monitor renal disease. Creatinine is a product of normal muscle breakdown. Urea is the waste product of breakdown of protein. The level of these substances rises in the blood as kidney function worsens.
Estimated GFR (eGFR): The laboratory or physician may calculate an estimated GFR using the information from a patient's blood work. It is not as accurate in patients younger than 18, pregnant patients, and those who are very muscular and those who are very overweight. It is important to be aware of one's estimated GFR and stage of chronic kidney disease. The physician uses the patient's stage of kidney disease to recommend additional testing and provide suggestions on management.
Electrolyte levels and acid-base balance: Kidney dysfunction causes imbalances in electrolytes, especially potassium, phosphorus, and calcium. High potassium (hyperkalemia) is a particular concern. The acid-base balance of the blood is usually disrupted as well.
Decreased production of the active form of vitamin D can cause low levels of calcium in the blood. Inability of failing kidneys to excrete phosphorus causes its levels in the blood to rise. Testicular or ovarian hormone levels may also be abnormal.
Blood cell counts: Because kidney disease disrupts blood cell production and shortens the survival of red cells, the red blood cell count and hemoglobin may be low (anemia). Some patients may also have iron deficiency due to blood loss in their gastrointestinal system. Other nutritional deficiencies may also impair the production of red cells.
Ultrasound: Ultrasound is often used in the diagnosis of kidney disease. An ultrasound is a noninvasive type of imaging test. In general, kidneys are shrunken in size in chronic kidney disease, although they may be normal or even large in size in cases caused by adult polycystic kidney disease, diabetic nephropathy, and amyloidosis. Ultrasound may also be used to diagnose the presence of urinary obstruction, kidney stones and also to assess the blood flow into the kidneys.
Biopsy: A sample of the kidney tissue (biopsy) is sometimes required in cases in which the cause of the kidney disease is unclear. Usually, a biopsy can be collected with local anesthesia by introducing a needle through the skin into the kidney. This is usually done as an outpatient procedure, though some institutions may require an overnight hospital stay.