The kidney has several important functions, including getting rid of waste, maintaining the balance of water and various substances in the body, and producing hormones like vitamin D, erythropoietin, and renin. One unique role of the kidney is to filter out nitrogenous waste. When we deal with kidney problems, we mainly focus on issues related to this filtration process rather than individual ion or molecule handling.
For instance, someone might have trouble excreting acid, but we'd assess it in the context of metabolic acidosis, which has various causes beyond the kidney. In contrast, acute and chronic kidney diseases specifically concern problems with the kidney's filtering function. When this filtration is impaired, other kidney functions like hormone production and maintaining electrolyte balance may also be affected.
Diseases affecting the kidney's tubules can impact glomerular filtration rates and lead to acute or chronic kidney problems. Sometimes, early kidney disease only shows as changes in the quality of filtration, like having too much protein in the urine (albuminuria), rather than changes in the quantity of filtration and increased waste concentration. Even when the glomerular filtration rate (GFR) is normal, certain kidney issues can cause problems like blood in the urine without high protein levels.
In this chapter, we discuss how to approach patients with acute kidney problems, different types of kidney syndromes (nephrotic and nephritic), tubulointerstitial diseases, vascular issues in the kidney, papillary necrosis, and chronic kidney disease.
Pathobiology
The kidneys contain about 2 million filters called glomeruli, which typically filter around 180 liters of fluid each day. These filters are selective and prevent cells and large proteins (over 60 kD in size) from entering the filtered fluid. Smaller proteins are sometimes filtered and then reabsorbed in the kidney's tubules, keeping the urine protein concentration low. When there's a problem with the quantity or quality of this filtration, it indicates kidney disease.
In kidney diseases, the normal glomerular filtration rate (GFR) can decrease suddenly within hours to days in cases of acute kidney injury or gradually over months to years in chronic kidney disease. An abrupt drop in GFR is the main factor for diagnosing acute kidney injury, but unusual urine findings can help identify the cause of the injury. In chronic kidney disease, early stages might only show signs like protein in the urine (ranging from very small amounts to a lot) and abnormal urine findings, such as a few cells or even visible blood or pus. As chronic kidney disease progresses, GFR continues to decline, and in severe cases, dialysis or transplantation becomes necessary to manage uremia.
To assess how well the kidneys are working, one of the most accurate methods is to measure the Glomerular Filtration Rate (GFR) using specialized markers. However, these tests are costly and time-consuming, so they’re not used routinely. Instead, doctors commonly estimate GFR using methods like serum creatinine concentration, calculated creatinine clearance, and equations based on serum creatinine.
The kidneys play crucial roles in filtering waste products, regulating various substances in the body (like water, salts, and acids), and producing hormones like vitamin D, erythropoietin, and renin. Kidneys filter out waste products from the blood, which is essential for maintaining good health. When we talk about kidney diseases, we mostly focus on problems related to this filtering process rather than isolated issues with processing individual substances.
For instance, a patient might have a specific problem with their kidneys’ ability to handle acids. In such cases, doctors evaluate them for metabolic acidosis, which is a condition where the body has too much acid. Kidney disease is often categorized as acute kidney injury or chronic kidney disease, which both pertain to issues with the kidney’s filtration function. When filtration is impaired, other kidney functions, like hormone production and electrolyte balance, can also be affected.
Diseases affecting the kidney tubules, such as acute tubular necrosis and tubulointerstitial disease, can reduce the rate of glomerular filtration, leading to acute or chronic kidney problems. In some cases of early chronic kidney disease, the decrease in filtration quality (like the presence of albuminuria in urine) may be more noticeable than a reduction in the quantity of filtrate with increased nitrogenous waste. Sometimes, despite kidney filtration issues that cause significant proteinuria (excess protein in urine), the glomerular filtration rate (GFR) may still appear normal. Problems with the kidney’s filter can also allow certain cells like red blood cells (RBCs) to pass into the urine, which can be seen in conditions like the acute nephritic syndrome.
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In summary, kidney function is crucial for various bodily processes, and assessing it is essential for diagnosing and managing kidney diseases. Estimating GFR through serum creatinine levels is common, but it has limitations, as creatinine levels can be influenced by factors like muscle mass, age, and gender. An increase in serum creatinine over time can indicate kidney problems, and even a small rise can signify a significant decrease in GFR.
Approach to Diagnosis:
Doctors usually don’t recommend routine kidney screening for healthy adults without kidney disease risks. Kidney issues are often found when looking into other health problems. To assess kidney function, doctors follow a structured process.
They start with the serum creatinine level and GFR, which gives an estimate of kidney function. In some cases, they might confirm results using additional tests like cystatin C or a formal creatinine clearance test with a 24-hour urine collection. Creatinine clearance measures how fast your kidneys filter waste from your blood. It’s calculated using a formula:
Creatinine Clearance (CCr) = (Urine Creatinine × Urine Flow Rate) / Plasma Creatinine
This method tends to overestimate GFR by about 10% because creatinine isn’t just filtered; it’s also secreted into urine by kidney tubules. Collecting urine for 24 hours to calculate creatinine clearance can be tricky for patients and prone to errors.
Due to these challenges, doctors often rely on equations to estimate GFR based on simpler clinical data and lab results. The commonly used ones are the Cockcroft-Gault, the Modification of Diet in Renal Disease (MDRD) Study, and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations. The choice depends on the situation.
The Cockcroft-Gault equation can be problematic when estimating GFR for individuals with unusual body weight. The MDRD equations, while accessible, tend to underestimate GFR when creatinine levels are high, possibly leading to incorrect chronic kidney disease diagnoses. The CKD-EPI equation is considered more precise, especially for higher GFR values. Cystatin C may be useful for patients with upper-normal creatinine levels, but it doesn’t replace estimated GFR for most clinical needs.
Urinalysis is a valuable test but is sometimes ordered unnecessarily for healthy individuals. It checks the color and appearance of urine and involves using special dipsticks and microscopic analysis.
•Urine Color: Normally, urine is yellow due to certain pigments. Abnormal colors can indicate various conditions.
•Specific Gravity: This measures urine concentration and can be affected by substances like glucose or dye. A specific gravity of 1.010, called isosthenuria, is linked to chronic kidney disease.
•Urine pH: Typically, urine is slightly acidic (pH 5). It can become alkaline after meals or in vegetarian diets. High pH might signal infection or certain kidney issues.
•Glucose: Detected using dipsticks, it can indicate conditions like diabetes or kidney tubule problems.
•Protein: Dipsticks mainly detect albumin. Different levels correspond to different amounts of protein in urine. Microalbuminuria is an early sign of kidney issues.
•Hemoglobin: Dipsticks use the peroxidase activity of hemoglobin. It’s sensitive for detecting blood, even in small amounts. Persistent microscopic blood can be a risk factor for kidney disease or other issues.
•Leukocytes: Detected by leukocyte esterase, it suggests infections or inflammation.
•Urine Sediment: Examines various particles in urine.
•Hematuria: Red blood cells (RBCs) in urine can originate from various places. Dysmorphic RBCs indicate glomerular issues.
•Pyuria: White blood cells (WBCs) are common in urinary tract infections but can also signal other problems.
•Casts and Crystals: Casts are formed in kidney tubules and can indicate different conditions. Crystals can be normal or hint at underlying issues.
Remember, urinalysis is a helpful tool but needs proper interpretation by a healthcare professional for an accurate diagnosis.