Clinical laboratory nephrology: Laboratory Tests for Chronic Kidney Disease: What You Need to Know

1. What is Chronic Kidney Disease (CKD) and why is it important to diagnose it early?

chronic kidney disease (CKD) is a condition that affects the function of the kidneys, which are responsible for filtering waste and excess fluid from the blood, maintaining the balance of electrolytes and hormones, and producing urine. CKD can be caused by various factors, such as diabetes, high blood pressure, glomerulonephritis, polycystic kidney disease, and urinary tract infections. CKD can progress over time and lead to serious complications, such as cardiovascular disease, anemia, bone disease, and kidney failure. Therefore, it is important to diagnose CKD early and monitor its progression and treatment.

To diagnose CKD, several laboratory tests are used to assess the kidney function and damage. These tests include:

- Serum creatinine: Creatinine is a waste product of muscle metabolism that is filtered by the kidneys. The level of creatinine in the blood reflects the glomerular filtration rate (GFR), which is the rate at which the kidneys filter the blood. A high serum creatinine indicates a low GFR and impaired kidney function. The normal range of serum creatinine is 0.6 to 1.2 mg/dL for men and 0.5 to 1.1 mg/dL for women.

- Estimated glomerular filtration rate (eGFR): eGFR is a calculation based on the serum creatinine, age, sex, and race of the patient. It estimates how much blood the kidneys can filter in a minute. The normal range of eGFR is 90 to 120 mL/min/1.73 m^2. An eGFR below 60 mL/min/1.73 m^2 for three months or more indicates CKD. An eGFR below 15 mL/min/1.73 m^2 indicates kidney failure.

- Blood urea nitrogen (BUN): Urea is another waste product of protein metabolism that is filtered by the kidneys. The level of urea in the blood reflects the balance between the production and excretion of urea. A high BUN indicates a high urea production (such as in a high-protein diet or bleeding) or a low urea excretion (such as in kidney disease or dehydration). The normal range of BUN is 7 to 20 mg/dL.

- Urine albumin-to-creatinine ratio (UACR): Albumin is a protein that is normally present in the blood but not in the urine. The presence of albumin in the urine indicates damage to the glomeruli, which are the tiny filters in the kidneys that prevent the leakage of large molecules. The UACR measures the amount of albumin in a urine sample compared to the amount of creatinine. A high UACR indicates increased albuminuria and kidney damage. The normal range of UACR is less than 30 mg/g. A UACR of 30 to 300 mg/g indicates microalbuminuria, and a UACR of more than 300 mg/g indicates macroalbuminuria.

- Urinalysis: Urinalysis is a test that examines the physical, chemical, and microscopic properties of the urine. It can detect the presence of blood, protein, glucose, ketones, bacteria, and other substances in the urine that may indicate kidney disease or infection. A normal urinalysis should not show any abnormal findings, such as hematuria (blood in the urine), proteinuria (protein in the urine), glycosuria (glucose in the urine), ketonuria (ketones in the urine), or pyuria (pus in the urine).

These laboratory tests can help diagnose CKD and determine its stage, severity, and cause. They can also help monitor the response to treatment and the risk of complications. By performing these tests regularly, patients and health care providers can detect CKD early and prevent or delay its progression and complications.

2. How to classify the severity of kidney damage and function using the GFR and ACR tests?

One of the main goals of clinical laboratory nephrology is to diagnose and monitor chronic kidney disease (CKD), a condition that affects about 15% of the adult population worldwide. CKD is characterized by a progressive loss of kidney function over time, which can lead to serious complications such as cardiovascular disease, anemia, bone disorders, and end-stage renal disease (ESRD). To assess the severity of kidney damage and function in CKD, two important tests are used: the glomerular filtration rate (GFR) and the albumin-to-creatinine ratio (ACR). These tests provide valuable information about the stage of CKD and the risk of adverse outcomes. Here are some key points to know about these tests and how they are used to classify CKD:

- The GFR is a measure of how well the kidneys filter waste and excess fluid from the blood. It is calculated using a formula that takes into account the level of creatinine, a waste product that is normally removed by the kidneys, in the blood. The GFR is expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m2). A normal GFR is above 90 mL/min/1.73 m2, while a GFR below 60 mL/min/1.73 m2 for three months or more indicates CKD. A GFR below 15 mL/min/1.73 m2 indicates ESRD, which requires dialysis or kidney transplantation to sustain life.

- The ACR is a measure of how much albumin, a type of protein, is present in the urine. Albumin is normally retained by the kidneys, but when the kidneys are damaged, they may leak albumin into the urine. The ACR is calculated by dividing the amount of albumin in a urine sample by the amount of creatinine in the same sample. The ACR is expressed in milligrams of albumin per gram of creatinine (mg/g). A normal ACR is below 3 mg/g, while an ACR above 3 mg/g indicates kidney damage. An ACR above 30 mg/g indicates significant kidney damage and an increased risk of cardiovascular disease and death.

- The stages of CKD are based on the combination of the GFR and the ACR values. There are five stages of CKD, ranging from stage 1 (mild) to stage 5 (severe). Each stage has a different prognosis and management plan. The following table summarizes the stages of CKD and their corresponding GFR and ACR values:

| Stage | Description | GFR (mL/min/1.73 m2) | ACR (mg/g) |

| 1 | Kidney damage with normal or high GFR | ≥ 90 | > 3 |

| 2 | Kidney damage with mildly decreased GFR | 60-89 | > 3 |

| 3 | Moderately decreased GFR | 30-59 | Any |

| 4 | Severely decreased GFR | 15-29 | Any |

| 5 | Kidney failure or ESRD | < 15 | Any |

- For example, a person with a GFR of 45 mL/min/1.73 m2 and an ACR of 25 mg/g has stage 3 CKD. This means that their kidneys are moderately impaired and they have a moderate risk of developing complications. Their treatment plan may include medications to control blood pressure, blood sugar, and cholesterol, as well as lifestyle changes such as dietary modifications, exercise, and smoking cessation. They may also need to undergo regular blood and urine tests to monitor their kidney function and other parameters.

3. What are the common conditions and lifestyle factors that can lead to or worsen CKD?

Chronic kidney disease (CKD) is a condition that affects the function and structure of the kidneys over time. It can lead to serious complications such as cardiovascular disease, kidney failure, and death. CKD is often asymptomatic in its early stages, which makes it difficult to detect and treat. Therefore, it is important to understand the factors that can cause or worsen CKD and take preventive measures to protect your kidney health.

Some of the common factors that can increase the risk of developing or worsening CKD are:

- High blood pressure: This is the second most common cause of CKD after diabetes. High blood pressure damages the blood vessels in the kidneys and reduces their ability to filter waste and fluid from the blood. Over time, this can lead to scarring and loss of kidney function. According to the National Kidney Foundation, about 1 in 5 adults with high blood pressure has CKD. The recommended blood pressure for people with CKD is less than 130/80 mmHg.

- Diabetes: This is the leading cause of CKD in the world. Diabetes affects the way the body uses glucose (sugar) and can cause high levels of glucose in the blood. This can damage the nerves and blood vessels in the kidneys and impair their function. About 1 in 3 adults with diabetes has CKD. The recommended blood glucose level for people with CKD is between 70 and 130 mg/dL before meals and less than 180 mg/dL after meals.

- Glomerulonephritis: This is a group of diseases that cause inflammation and damage to the glomeruli, the tiny filters in the kidneys that remove waste and excess fluid from the blood. Glomerulonephritis can be caused by infections, autoimmune disorders, genetic mutations, or drugs. It can result in proteinuria (protein in the urine), hematuria (blood in the urine), reduced urine output, and kidney failure. Glomerulonephritis is the third most common cause of CKD in the world.

- Polycystic kidney disease (PKD): This is a genetic disorder that causes multiple cysts (fluid-filled sacs) to grow in the kidneys. The cysts can enlarge and compress the normal kidney tissue, reducing its function. PKD can also cause high blood pressure, kidney stones, urinary tract infections, and kidney failure. PKD affects about 1 in 500 people worldwide and is the fourth most common cause of CKD.

- Urinary tract obstruction: This is a condition that blocks the flow of urine from the kidneys to the bladder or from the bladder to the outside of the body. Urinary tract obstruction can be caused by kidney stones, enlarged prostate, tumors, or congenital anomalies. It can cause pressure and damage to the kidneys and increase the risk of infection and kidney failure. Urinary tract obstruction can affect people of any age and gender, but it is more common in men over 50 years old.

- Lifestyle factors: Some habits and behaviors that can negatively affect the kidney health are smoking, drinking alcohol, using illicit drugs, eating a high-salt or high-protein diet, being overweight or obese, and being physically inactive. These factors can increase the blood pressure, blood glucose, cholesterol, and uric acid levels, which can damage the kidneys and other organs. To prevent or delay CKD, it is recommended to quit smoking, limit alcohol intake, avoid drug abuse, eat a balanced and kidney-friendly diet, maintain a healthy weight, and exercise regularly.

4. How to eat well and maintain a healthy weight with CKD and what foods to avoid or limit?

One of the most important aspects of managing chronic kidney disease (CKD) is to follow a healthy diet that supports your kidney function and overall well-being. A balanced diet can help you control your blood pressure, blood sugar, and cholesterol levels, which are essential for preventing further damage to your kidneys and other organs. It can also help you maintain a healthy weight, which reduces the risk of complications such as cardiovascular disease and diabetes. However, not all foods are beneficial for people with CKD, and some may even worsen your condition. Therefore, it is crucial to know what foods to eat and what foods to avoid or limit when you have CKD.

Here are some general guidelines for a kidney-friendly diet:

- Limit your sodium intake. Sodium is a mineral that helps regulate fluid balance and blood pressure in the body. However, too much sodium can cause fluid retention, swelling, and high blood pressure, which can strain your kidneys and heart. The recommended daily intake of sodium for people with CKD is less than 2,000 mg, which is equivalent to about one teaspoon of salt. To reduce your sodium intake, you should avoid or limit processed foods, canned foods, salted snacks, sauces, condiments, and fast foods, which are often high in sodium. You should also use herbs, spices, lemon juice, vinegar, or salt-free seasonings to flavor your food instead of salt.

- Limit your potassium intake. Potassium is a mineral that helps regulate nerve and muscle function, including the heartbeat. However, too much potassium can cause irregular heart rhythms and even cardiac arrest, especially if your kidneys are not able to filter it out of your blood. The recommended daily intake of potassium for people with CKD is between 2,000 and 3,000 mg, depending on your blood level and your doctor's advice. To limit your potassium intake, you should avoid or limit high-potassium foods, such as bananas, oranges, tomatoes, potatoes, avocados, spinach, beans, nuts, seeds, chocolate, and coffee. You should also rinse canned fruits and vegetables before eating them, and cook high-potassium vegetables in water and drain them well to reduce their potassium content.

- Limit your phosphorus intake. Phosphorus is a mineral that helps build strong bones and teeth, and also plays a role in energy metabolism and cell function. However, too much phosphorus can cause bone loss, itching, and calcification of soft tissues, such as the blood vessels and the heart, if your kidneys are not able to remove it from your blood. The recommended daily intake of phosphorus for people with CKD is between 800 and 1,200 mg, depending on your blood level and your doctor's advice. To limit your phosphorus intake, you should avoid or limit high-phosphorus foods, such as dairy products, meat, poultry, fish, eggs, nuts, seeds, bran, oatmeal, and cola drinks. You should also avoid or limit foods that contain phosphorus additives, such as baking powder, cheese spreads, and processed meats, which are often found in the ingredient list as "phos" or "phosphate".

- Limit your protein intake. Protein is a macronutrient that helps build and repair muscles, organs, and tissues in the body. However, too much protein can increase the workload of your kidneys and cause more waste products to accumulate in your blood, which can worsen your CKD. The recommended daily intake of protein for people with CKD is between 0.6 and 0.8 grams per kilogram of body weight, depending on your stage of CKD and your doctor's advice. To limit your protein intake, you should choose lean sources of protein, such as chicken, turkey, fish, eggs, and soy, and limit your portion sizes to about 3 ounces per meal, which is equivalent to the size of a deck of cards. You should also avoid or limit high-protein foods, such as red meat, pork, organ meats, cheese, and milk, which are also high in phosphorus and sodium.

- Increase your fiber intake. Fiber is a type of carbohydrate that helps lower your cholesterol and blood sugar levels, and also promotes bowel regularity and satiety. Fiber can also help bind some of the waste products in your intestines and prevent them from being absorbed into your blood, which can improve your kidney function and reduce inflammation. The recommended daily intake of fiber for people with CKD is between 25 and 38 grams, depending on your age and gender. To increase your fiber intake, you should eat more fiber-rich foods, such as fruits, vegetables, whole grains, beans, lentils, and nuts. You should also drink plenty of water to help fiber move through your digestive system and prevent constipation.

- Drink enough fluids. Fluids are essential for maintaining hydration, blood volume, and blood pressure in the body. However, too much or too little fluid can cause problems for people with CKD, depending on their urine output and fluid retention. If you have a low urine output and a high fluid retention, you may need to limit your fluid intake to prevent swelling, shortness of breath, and high blood pressure. If you have a high urine output and a low fluid retention, you may need to increase your fluid intake to prevent dehydration, low blood pressure, and electrolyte imbalance. The recommended daily intake of fluids for people with CKD varies from person to person, depending on their weight, urine output, fluid retention, and other factors. You should consult your doctor or dietitian to determine your individual fluid needs and monitor your fluid intake and output regularly. You should also avoid or limit caffeinated, alcoholic, and sugary drinks, which can increase your fluid intake and affect your kidney function.

By following these dietary guidelines, you can help protect your kidneys and improve your quality of life with CKD. However, keep in mind that these are general recommendations and may not apply to everyone. Your diet may need to be adjusted according to your stage of CKD, your blood test results, your medications, and your personal preferences. Therefore, it is important to work with your doctor and dietitian to create a personalized meal plan that meets your nutritional needs and goals. They can also provide you with more specific advice and tips on how to eat well and maintain a healthy weight with CKD.