Kidney Function: Importance of Kidneys, Kidney Problems, Values, Types of Tests, and Treatment of Early Kidney Disease

Your kidneys play several vital roles in maintaining your health. One of its most important jobs is to filter waste materials from the blood and expel them from the body in the form of urine.

You have two kidneys on each side of your spine that are about the size of a human fist. They are located behind your abdomen and under your rib cage.

The kidneys also help control the levels of water and several essential minerals in the body. In addition, they are essential for the production of:

  • Vitamin D.
  • Red blood cells.
  • Hormones that regulate blood pressure.

If your doctor thinks that your kidneys may not work properly, you may need kidney function tests. These are simple blood and urine tests that can identify problems with your kidneys.

You may also need kidney function tests if you have other conditions that can damage the kidneys, such as diabetes or high blood pressure . They can help doctors manage these conditions.

They involve the measurement of various substances in both blood and urine to estimate the filtration efficiency of the kidneys.

Symptoms of kidney problems

Symptoms that may indicate a problem with the kidneys include:

  • High blood pressure
  • Blood in the urine. Frequent urges to urinate
  • Difficulty starting to urinate.
  • Pain when urinating
  • Swelling of the hands and feet due to a build-up of fluids in the body.

A single symptom may not mean something serious. However, when they occur simultaneously, these symptoms suggest that your kidneys are not working properly. Kidney function tests can help determine why.

Risk factors for chronic kidney disease include, but are not limited to: older age, male gender, race, hypertension, diabetes mellitus, smoking, family history of kidney disease, proteinuria, nephrotoxin exposure, and atherosclerosis.

Renal function values

Accurate measurement of kidney function is vital for any patient with suspected chronic kidney disease (CKD). More than 20 million adults in the United States, or more than 10% of the adult population, have chronic kidney disease.

Kidney function is commonly assessed by measurements of blood urea nitrogen, serum creatinine level (SCr), and creatinine clearance.

The serum creatinine level is relatively insensitive to the detection of early changes in glomerular filtration rate (GFR), and its absolute level is influenced by muscle mass and age.

The glomerular filtration rate can be considered as “the volume of cleared plasma of an ideal substance per unit of time”, expressed in ml / min, standardized for the body surface area.

A glomerular filtration rate of 60-89 ml / ml / 1.73 m2 is considered normal. Overestimation of glomerular filtration rate can lead to drug toxicity, and underestimation of glomerular filtration rate can lead to subtherapeutic drug levels.

Potentially life-changing clinical decisions, such as when to start dialysis, are also based in part on measuring the glomerular filtration rate.

In early kidney disease, an initial decrease in glomerular filtration rate can lead to a slight increase (0.2 mg / dL or less) in serum creatinine concentration due to increased proximal tubular creatinine secretion.

Consequently, patients with a glomerular filtration rate of 60 to 80 ml / min may have a serum creatinine concentration of approximately 1.0 mg / dl.

However, once serum creatinine levels exceed 1.5 to 2.0 mg / dL, changes in serum creatinine level correlate well with changes in glomerular filtration rate.

Some guidelines recommend that planning for patients’ future dialysis begins when their estimated glomerular filtration rate (eGFR) is <30 ml / min / 1.73 m2 and decreases.

Dialysis usually begins when patients become symptomatic with an estimated glomerular filtration rate of <15 ml / min / 1.73 m2.

The glomerular filtration rate calculated by measuring the clearance of inulin by the kidney is considered “gold standard” because inulin is a physiologically inert substance, that is, it is freely filtered by the kidneys without absorption or secretion by the tubules.

In clinical practice, accurate measurement of the glomerular filtration rate by inulin clearance is possible, but not practical as this process is costly, time-consuming, and unskillful.

For this reason, there has been a constant effort to develop an equation that can reliably estimate glomerular filtration rate from blood and urine biochemical markers, such as creatinine.

These are fast and profitable. However, with age there is a change in both kidney physiology and muscle mass, which can affect estimates of the glomerular filtration rate calculation.

This means that estimates of the glomerular filtration rate estimate may be less reliable in older patients and therefore may adversely affect their clinical care.

Creatinine clearance

Assessment of renal function is most easily accomplished by measuring creatinine clearance in 24-hour urine collections; the required 24-hour urine collection is cumbersome, and collection errors can lead to unreliable estimates.

While previously used to measure glomerular filtration rate, creatinine clearance has now fallen out of favor due to the required 24-hour urine collection period. This is so demanding that imprecise collection volumes are the result. Measurements can also vary by up to 25%.

Although active creatinine secretion results in falsely high urine creatinine, this error is stable and predictable, providing an accurate picture of glomerular filtration rate.

Predictive equations for estimating glomerular filtration rate offer a rapid alternative method to assess kidney function based on serum creatinine concentration and anthropometric data.

To this end, the Cockcroft-Gault equations and diet modification in kidney disease (MDRD) are widely used in clinical practice, although both lack precision when the glomerular filtration rate is 60 ml / min / 1.73 m2 or more.

Historically, serum creatinine has been used as a surrogate for glomerular filtration rate based on the assumption that it is produced, filtered, and secreted in a steady state.

The Cockcroft-Gault equation was developed to estimate creatinine clearance, with the assumption that creatinine clearance was a direct measure of glomerular filtration rate.

Cockcroft-Gault uses serum creatinine adjusted for age, weight, serum creatinine, and gender to obtain creatinine clearance. Cockcroft-Gault was the most widely used kidney function index from its introduction in 1976 until the development of diet modification in the kidney disease equation.

Cockcroft-Gault has been validated against glomerular filtration rate measured using I-ihalamate, which showed that the Cockcroft-Gault equation overestimated glomerular filtration rate by 16%.

The reason for this and the key limitations of using creatinine clearance and serum creatinine as estimates of glomerular filtration rate are that there are actually many factors that can affect creatinine metabolism in muscles and the rate of creatinine secretion. in the tubules.

More clearly, creatinine is affected by muscle mass, which in turn changes with age, and also between genders and ethnic groups.

Other factors that can affect creatinine production and secretion and that may also differ in older versus younger patients include dietary protein intake, malnutrition, and prescribed medication.

Studies have also shown that creatinine clearance overestimates the glomerular filtration rate due to creatinine secretion from the tubules in normal individuals.

In patients with chronic kidney disease, there is increased extrarenal creatinine clearance and decreased creatinine clearance leading to an overestimation of the glomerular filtration rate of serum creatinine.

Changes in renal function are more important than absolute values ​​of renal function, and reproducible changes in serum creatinine concentration (increase from 20% to 30% or more).

Or the glomerular filtration rate (deterioration of 10% or more from baseline) are worrisome because they indicate significant loss of kidney function.

Persistently elevated serum creatinine levels (2.0mg / dL or more) at the time of response to immunosuppressive treatment are associated with a positive likelihood ratio of 10.8 for later development of end-stage renal disease (ESRD). in English).

In contrast, a reduction in serum creatinine concentration (even within the ‘normal’ range of values) at 6 months after induction therapy is associated with a higher probability of a favorable long-term renal outcome (odds ratio, 14.9).

Renal function should be assessed in all patients with known kidney disease, diabetes, liver dysfunction, and those with ileus, who are parenterally fed or who are likely to have intravenous fluid administration and perioperative fluid loss.

Types of evidence

To assess your kidney function, your doctor will order a series of tests that can estimate your glomerular filtration rate that tells your doctor how fast your kidneys are clearing waste from your body.

Urine analysis

A urinalysis detects the presence of protein and blood in the urine. There are many possible reasons for protein in the urine, and not all are related to the disease.

The infection increases protein in the urine, but so does intense physical training. Your doctor may want to repeat this test after a few weeks to see if the results are similar.

Your doctor may also ask you to provide a 24-hour urine collection sample. This can help doctors see how quickly a waste product called creatinine is being cleared from your body. Creatinine is a breakdown product of muscle tissue.

Serum creatinine test

This blood test looks to see if creatinine is building up in the blood. The kidneys generally completely filter creatinine from the blood. A high creatinine level suggests a kidney problem.

According to the National Kidney Foundation (NKF), a creatinine level greater than 1.2 milligrams / deciliter (mg / dL) for women and 1.4 mg / dL for men is a sign of a kidney problem

Blood urea nitrogen

The blood urea nitrogen (BUN) test also checks for waste products in your blood. Blood urea nitrogen tests measure the amount of nitrogen in the blood. Urea nitrogen is a breakdown product of protein.

However, not all elevated blood urea nitrogen tests are due to kidney damage. Common medications, which include large doses of aspirin and some types of antibiotics, can also increase blood urea nitrogen.

It is important that you inform your doctor about any medications or supplements that you take regularly. You may need to stop certain medications for a few days before the test.

A normal blood urea nitrogen level is between 7 and 20 mg / dL. A higher value could suggest several different health problems.

Estimated glomerular filtration rate

This test calculates how well your kidneys are filtering waste. The test determines speed by looking at factors such as:

  • Test results, specifically creatinine levels.
  • Age.
  • Gender.
  • Race.
  • Height.
  • Weight.

As mentioned above any result less than 60 milliliters / minute / 1.73m2 can be a warning sign of kidney disease.

Importance of kidney function tests

Kidney function tests are important to:

  • Early detection of kidney disease.
  • Calibration of doses of various drugs that are excreted or metabolized in the kidneys.
  • Indicating the appropriate time to start dialysis.
  • Evaluation of renal status in donors and recipients of heart and kidney transplants.

An individual who is at risk for kidney failure is advised to have periodic examinations to detect any deterioration in kidney function.

How the tests are performed

Kidney function tests generally require a 24-hour urine sample and a blood test.

24-hour urine sample

Mentioned above the 24 hour urine sample is a creatinine clearance test. It gives your doctor an idea of ​​how much creatinine your body expels in a single day.

On the day the test begins, urinate into the toilet as you normally would when you wake up.

For the rest of the day and night, urinate into a special container provided by your doctor. Keep the container covered and refrigerated during the collection process. Be sure to clearly label the container and tell other family members why it is in the refrigerator.

On the morning of the second day, urinate into the container when you wake up. This completes the 24 hour collection process.

Follow your doctor’s instructions on where to drop the sample. You may need to return it to your doctor’s office or a lab.

Blood samples

Blood urea nitrogen and serum creatinine tests require blood samples taken in a laboratory or doctor’s office.

The technician who draws the blood first ties an elastic band around the upper arm. This makes the veins stand out. The technician then cleans the area over the vein. They pass a hollow needle through your skin and into the vein.

The blood will flow back into a test tube that will be sent for testing. You may feel a sharp prick or prick when the needle enters your arm.

The technician will place gauze pads and a bandage over the puncture site after the test. The area around the puncture may develop a bruise in the next few days. However, you should not feel severe or prolonged pain.

Treatment of early kidney disease

Your doctor will focus on treating the underlying condition if tests show early kidney disease. Your doctor will prescribe medication to control your blood pressure if tests indicate hypertension . They will also suggest lifestyle and diet modifications.

If you have diabetes, your doctor may want you to see an endocrinologist. This type of doctor specializes in metabolic diseases and can help ensure that you have the best possible blood glucose control.

If there are other causes of your abnormal kidney function tests, such as kidney stones and overuse of pain relievers, your doctor will take appropriate steps to control those disorders.

Abnormal test results mean that you will probably need regular kidney function tests in the next few months. This will help your doctor monitor your condition.