A nephrologist uses a combination of blood tests, urine tests, imaging, and sometimes a kidney biopsy to evaluate how well your kidneys are working and what might be causing problems. Most of these tests are straightforward, and many can be done at a routine lab visit. Here’s what to expect.
Blood Tests for Kidney Function
The first thing a nephrologist will order is blood work to measure how effectively your kidneys are filtering waste. The most important number here is your eGFR, or estimated glomerular filtration rate. This tells your doctor how many milliliters of blood your kidneys clean per minute. An eGFR of 60 or higher is normal. Below 60 suggests kidney disease, and 15 or lower points to kidney failure.
Your blood will also be checked for creatinine and blood urea nitrogen (BUN), both waste products that build up when your kidneys aren’t filtering properly. These values feed into the eGFR calculation, so you’ll typically see all three reported together.
Electrolyte and Mineral Panels
Your kidneys regulate the balance of key minerals in your blood, so a nephrologist will check levels of sodium, potassium, calcium, and phosphorus. When kidney function declines, these can drift out of range in ways that affect your whole body. Your kidneys handle roughly 90 to 95 percent of potassium regulation, for example, which is why dangerous potassium levels are one of the hallmarks of advanced kidney disease.
Calcium and phosphorus deserve special attention. Low calcium has been linked to faster progression toward kidney failure, while excess phosphorus pulls calcium from your bones and can deposit in blood vessels and organs, raising the risk of heart attack and stroke. These aren’t abstract lab numbers. They directly shape treatment decisions, including dietary changes and medications your nephrologist may recommend.
Whether you need to fast before blood work depends on which specific tests are ordered. If fasting is required, it typically means no food or drink (besides water) for 8 to 12 hours. You should also avoid chewing gum, smoking, and exercise during the fasting window. Ask your doctor’s office ahead of time so you can plan accordingly, and always confirm whether it’s okay to take your regular medications the morning of the test.
Urine Tests
Blood tests show how well your kidneys filter waste, but urine tests reveal whether they’re leaking things they shouldn’t be, particularly protein. A healthy kidney keeps protein in your blood. When protein spills into your urine, it’s an early warning sign of kidney damage.
The most common urine test a nephrologist orders is the urine albumin-to-creatinine ratio, or UACR. This measures a specific protein called albumin in a single urine sample. A result under 30 mg/g is normal. Between 30 and 300 mg/g indicates moderately increased protein loss, which is often the first detectable sign of kidney disease. Above 300 mg/g signals severe protein loss and more advanced damage. A related test, the urine protein-to-creatinine ratio (UPCR), captures a broader range of proteins and uses slightly different thresholds: normal is under 150 mg/g, moderate is 150 to 500, and severe is above 500.
These tests are central to staging chronic kidney disease. Your nephrologist will likely repeat them over time to track whether protein loss is stable, improving with treatment, or getting worse.
24-Hour Urine Collection
For kidney stone evaluation or a more detailed look at kidney function, you may be asked to collect all of your urine over a full 24 hours. You’ll be given a special container that needs to stay refrigerated throughout the collection period. The lab analyzes this sample for imbalances in minerals, salts, and other substances that promote stone formation.
Accuracy matters with this test. Forgetting to collect even one sample, going past the 24-hour window, or not keeping the container cold can skew results. Certain foods and drinks (coffee, tea, bananas, citrus fruits) and vigorous exercise can also affect accuracy, so your doctor may give you specific instructions about what to avoid during the collection day.
Kidney Imaging
A nephrologist will often order imaging to look at the physical structure of your kidneys. The most common starting point is an ultrasound, which is painless, uses no radiation, and can show whether your kidneys are the right size and in the right position. It also detects blockages, kidney stones, cysts, and tumors. A bladder ultrasound may be done at the same time to check for stones, pouches in the bladder wall, or other structural problems.
If the ultrasound raises questions, your nephrologist may order a CT scan (sometimes called a CT urogram). CT provides a more detailed picture and can identify stones, obstructions, infections, cysts, tumors, and traumatic injuries throughout the urinary tract.
MRI is reserved for specific situations. A type called magnetic resonance angiography (MRA) gives the most detailed view of the arteries supplying blood to your kidneys, which is important when narrowing of those arteries is suspected. Another type, magnetic resonance urography (MRU), is used to evaluate blood in the urine, monitor patients with a history of urinary tract cancers, or investigate recurrent urinary tract infections.
Kidney Biopsy
A kidney biopsy is not routine, but it becomes necessary when blood and urine tests show problems that can’t be diagnosed any other way. The most common reasons for a biopsy are persistent blood in your urine, too much protein in your urine, or unexplained decline in kidney function. It’s also used to evaluate a transplanted kidney that isn’t working properly, or to investigate an abnormal mass found on imaging.
The procedure is performed by a radiologist or nephrologist, usually at a hospital or outpatient center, and takes about an hour. You’ll lie face down (or face up if you have a transplanted kidney) while a doctor uses ultrasound guidance to insert a thin needle through your skin and into the kidney. You’ll receive a local anesthetic to numb the area, and often a mild sedative through an IV. The needle may be inserted more than once to collect enough tissue, and it can make a clicking or popping sound, which is normal.
Afterward, you’ll rest in a recovery room for several hours while staff monitors your blood pressure, pulse, and urine for signs of internal bleeding. Most people go home the same day, though an overnight stay is sometimes needed. The tissue samples are examined under a microscope for signs of inflammation, scarring, infection, or unusual protein deposits. This information helps your nephrologist pinpoint the exact type of kidney disease and choose the most effective treatment.
Genetic Testing
For certain inherited kidney conditions, a nephrologist may order genetic testing. This is most relevant for diseases like Alport syndrome and polycystic kidney disease, where identifying the specific genetic mutation can confirm the diagnosis, predict how the disease will progress, and guide treatment. Genetic testing also helps determine whether family members are at risk. It’s typically a blood test or cheek swab, and results are usually discussed in a follow-up appointment where your nephrologist can explain what the findings mean for your long-term care.