Assessment of Renal Function
There are several clinical laboratory tests that are useful in investigating and evaluating kidney function. Clinically, the most practical tests to assess renal function is to get an estimate of the glomerular filtration rate (GFR) and to check for proteinuria (albuminuria).
Glomerular Filtration Rate
The best overall indicator of the glomerular function is the glomerular filtration rate (GFR). GFR is the rate in milliliters per minute at which substances in plasma are filtered through the glomerulus; in other words, the clearance of a substance from the blood. The normal GFR for an adult male is 90 to 120 mL per minute. The characteristics of an ideal marker of GFR are as follows:
- It should appear endogenously in the plasma at a constant rate
- It should be freely filtered at the glomerulus
- It can be neither reabsorbed nor secreted by the renal tubule
- It should not undergo extrarenal elimination.
As no such endogenous marker currently exists, exogenous markers of GFR are used. Assessment of GFR using inulin, a polysaccharide, is considered the reference method for the estimation of GFR. It involves the infusion of inulin and then the measurement of blood levels after a specified period to determine the rate of clearance of inulin. Other exogenous markers used are radioisotopes such as chromium-51 ethylene-diamine-tetra-acetic acid (51 Cr-EDTA), and technetium-99-labeled diethylene-triamine-pentaacetate (99 Tc-DTPA). The most promising exogenous marker is the non-radioactive contrast agent, iohexol, especially in children.
The inconvenience associated with the use of exogenous markers, specifically that the testing has to be performed in specialized centers, and the difficulty to assay these substances, has encouraged the use of endogenous markers.
The most commonly used endogenous marker for the assessment of glomerular function is creatinine. The calculated clearance of creatinine is used to provide an indicator of GFR. This involves the collection of urine over a 24-hour period or preferably over an accurately timed period of 5 to 8 hours since 24-hour collections are notoriously unreliable. Creatinine clearance is then calculated using the equation:
C = clearance, U = urinary concentration, V = urinary flow rate (volume/time i.e. ml/min), and P = plasma concentration
Creatinine clearance should be corrected for body surface area. Improper or incomplete urine collection is one of the major issues affecting the accuracy of this test; hence timed collection is advantageous. Furthermore, due to tubular secretion, creatinine overestimates GFR by around 10% to 20%.
Creatinine is the by-product of creatine phosphate in muscle, and it is produced at a constant rate by the body. For the most part, creatinine is cleared from the blood entirely by the kidney. Decreased clearance by the kidney results in increased blood creatinine. The amount of creatinine produced per day depends on muscle bulk. Thus, there is a difference in creatinine ranges between males and females with lower creatinine values in children and those with decreased muscle bulk. Diet also influences creatinine values. Creatinine can change as much as 30% after the ingestion of red meat. As GFR increases in pregnancy, lower creatinine values are found in pregnancy. Additionally, serum creatinine is a later indicator of renal impairment-renal function is decreased by 50% before a rise in serum creatinine is observed.
Serum creatinine is also utilized in GFR estimating equations such as the Modified Diet in Renal Disease (MDRD) and the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation. These eGFR equations are superior to serum creatinine alone since they include race, age, and gender variables. GFR is classified into the following stages based on kidney disease.
Kidney Disease Improving Global Outcomes (KDIGO) stages of chronic kidney disease (CKD):
- Stage 1 GFR greater than 90 ml/min/1.73 m²
- Stage 2 GFR-between 60 to 89 ml/min/1.73 m²
- Stage 3a GFR 45 to 59 ml/min/1.73 m²
- Stage 3b GFR 30 to 44 ml/min/1.73 m²
- Stage 4 GFR of 15 to 29 ml/min/1.73 m²
- Stage 5-GFR less than 15 ml/min/1.73 m² (end-stage renal disease)
This provides an easier estimation of GFR without the collection of urine or the use of exogenous materials. However, as they utilize serum creatinine, they are also affected by the issues around serum creatinine measurement; hence the correction for the race, gender, and age is required.
Blood Urea Nitrogen (BUN)
Urea or BUN is a nitrogen-containing compound formed in the liver as the end product of protein metabolism and the urea cycle. About 85% of urea is eliminated via kidneys; the rest is excreted via the gastrointestinal (GI) tract. Serum urea levels increase in conditions where renal clearance decreases (in acute and chronic renal failure/impairment). Urea may also increase in other conditions not related to renal diseases such as upper GI bleeding, dehydration, catabolic states, and high protein diets. Urea may be decreased in starvation, low-protein diet, and severe liver disease. Serum creatinine is a more accurate assessment of renal function than urea; however, urea is increased earlier in renal disease.
The ratio of BUN: creatinine can be useful to differentiate pre-renal from renal causes when the BUN is increased. In pre-renal disease, the ratio is close to 20:1, while in intrinsic renal disease, it is closer to 10:1. Upper GI bleeding can be associated with a very high BUN to creatinine ratio (sometimes >30:1).
Cystatin C is a low-molecular-weight protein that functions as a protease inhibitor produced by all nucleated cells in the body. It is formed at a constant rate and freely filtered by the kidneys. Serum levels of cystatin C are inversely correlated with the glomerular filtration rate (GFR). In other words, high values indicate low GFRs, while lower values indicate higher GFRs, similar to creatinine. The renal handling of cystatin C differs from creatinine. While glomeruli freely filter both, once cystatin C is filtered, it is reabsorbed and metabolized by proximal renal tubules, unlike creatinine. Thus, under normal conditions, cystatin C does not enter the final excreted urine to any significant degree. Cystatin C is measured in serum and urine. The advantages of cystatin C over creatinine are that it is not affected by age, muscle bulk, or diet, and various reports have indicated that it is a more reliable marker of GFR than creatinine, particularly in early renal impairment. Cystatin C has also been incorporated into eGFR equations, such as the combined creatinine-cystatin KDIGO CKD-EPI equation.
Cystatin C concentration may be affected by the presence of cancer, thyroid disease, and smoking.
Albuminuria and Proteinuria
Albuminuria refers to the abnormal presence of albumin in the urine. Microalbumin, considered an obsolete term as there is no such biochemical molecule, is now referred to only as urine albumin. Albuminuria is used as a marker for the detection of incipient nephropathy in diabetics. It is an independent marker for the cardiovascular disease since it connotes increased endothelial permeability, and it is also a marker for chronic renal impairment. Urine albumin may be measured in 24-hour urine collections or early morning/random specimens as an albumin/creatinine ratio. The presence of albuminuria on two occasions with the exclusion of a urinary tract infection indicates glomerular dysfunction. The presence of albuminuria for three or more months is indicative of chronic kidney disease. Frank proteinuria is defined as greater than 300 mg per day of protein. Normal urine protein is up to 150 mg per day (30% albumin; 30% globulins; 40% Tamm Horsfall protein). Increased amounts of protein in the urine may be due to:
- Glomerular proteinuria: Caused by defects in permselectivity of the glomerular filtration barrier to plasma proteins (for example, glomerulonephritis or nephrotic syndrome)
- Tubular proteinuria: Caused by incomplete tubular reabsorption of proteins (for example, interstitial nephritis)
- Overflow proteinuria: Caused by increased plasma concentration of proteins (for example, multiple myeloma-Bence Jones protein, myoglobinuria)
- Urinary tract inflammation or tumor
Urine protein may be measured using either a 24-hour urine collection or random urine protein: creatinine ratio (early morning sample is preferred since it is a near representative of the 24-hour sample).
The KDIGO classification defines three stages of albuminuria:
- A1: Less than 30 mg/g creatinine
- A2: 30 to 300 mg/g creatinine
- A3: Greater than 300 mg/g creatinine
In nephrotic syndrome, urine protein excretion exceeds 3.5 g per day and is associated with edema, hypoalbuminemia, and hypercholesterolemia.
Tests of Tubular Function
The renal tubules play a vital role in the reabsorption of electrolytes, water, and maintaining acid-base balance. Electrolytes - sodium, potassium, chloride, magnesium, phosphate as well as glucose can be measured in urine. Measurement of urine osmolality allows for assessment of concentrating ability of urine tubules. A urinary osmolality higher than 750 mOsmol/Kg H2O implies a normal concentrating ability of tubules. A water deprivation test can be used to exclude nephrogenic diabetes insipidus. Also, in distal renal tubular acidosis (RTA), an ammonium chloride test can be used to confirm the diagnosis of distal RTA with failure to acidify the urine to a pH of less than 5.3. In Fanconi's syndrome, there is aminoaciduria, glycosuria, phosphaturia, and bicarbonate wasting (proximal RTA).
Urine analysis involves the assessment of urine characteristics to aid in disease diagnosis. It consists of physical observation, chemical, and microscopic examination. The physical inspection involves assessing color and clarity. The normal urine is straw-colored, while in the presence of dehydration, urine is darker in color. Red urine may indicate hematuria or porphyria or could represent the dietary intake of food like beets. Cloudy urine may be seen in the presence of pyuria due to urinary tract infection. Specific gravity is an indicator of the renal concentrating ability, which can be measured using refractometry or chemically by the use of urine dipstick. The physiological range for specific gravity is 1.003 to 1.030. Specific gravity is increased in concentrated urine and decreased in dilute urine.
Urine dipstick provides qualitative analysis of different analytes in urine using chemical analysis.
Dipstick uses dry chemistry methods to detect the presence of protein, glucose, blood, ketones, bilirubin, urobilinogen, nitrite, and leukocyte esterase. The dipstick can be performed as a point-of-care test. The color changes following interaction of the urine with the chemical reagents impregnated on the paper of the dipstick are compared to the color chart guide to interpret the results.
Analytes tested on urine dipstick-protein should not be detectable in healthy urine specimens. Bilirubin is not detected in normal urine. Glucose is not detected in healthy patients but may be seen in diabetes mellitus, pregnancy, and renal glycosuria when the renal threshold of 180 mg/dl is decreased. The presence of ascorbic acid (vitamin C) and some antibiotics may affect results. Blood may be present after renal tract injury or infection, with ascorbic acid causing a falsely negative result. Urine dipstick detects the globin portion of hemoglobin, and thus cannot detect the difference between the presence of myoglobin or hemoglobin in urine.
Additionally, both intact red blood cells (RBC) and hemoglobinuria are detected. The presence of "blood" on urine dipstick test with normal RBC indicates rhabdomyolysis and can help differentiate it from hematuria, where RBCs are also detected on the urine dipstick. In normal urine, RBC per high-power field is between 0 to 3 and white blood cells (WBC) between 0 to 5. Ketones are present in fasting, severe vomiting, and diabetic ketoacidosis. Urine dipstick only detects acetoacetate and acetone, not the ketone beta-hydroxybutyrate. Bilirubin is detected in the presence of conjugated hyperbilirubinemia. Urobilinogen may typically be present, but it is absent in conjugated hyperbilirubinemia and increased in the presence of prehepatic jaundice and hemolysis. Nitrite and leucocyte esterase are indicators of urinary tract infection. Some bacteria, for example, Enterobacteriaceae, convert nitrates to nitrites.
The microscopic analysis involves a wet-prep analysis of urine to assess the presence of cells, casts, and crystals as well as micro-organisms. Red blood casts usually denote glomerulonephritis, while white blood cell casts are consistent with pyelonephritis. The presence of white blood cells and WBC casts indicates infection; red blood cells indicate renal injury; RBC casts indicate tubular damage or glomerulonephritis. Hyaline casts consist of protein and may occur in glomerular disease. Fatty casts are seen in nephrotic syndrome. Crystals may also be identified in urine and are indicative of the following conditions:
- Triple phosphate crystals have the "coffin-lid" appearance and can be seen in alkaline urine and urinary tract infection.
- Uric acid crystals are needle-shaped and are associated with gout.
- Oxalate crystals are envelope-shaped and are present in ethylene glycol poisoning or primary and secondary hyperoxaluria.
- Cystine crystals are hexagonal and are observed in cystinuria.
The best specimen for urine analysis is a freshly voided midstream urine. Midstream urine is less likely to be contaminated by commensal bacteria and epithelial cells.
Acute versus Chronic Renal Impairment
Acute renal impairment or acute kidney injury (AKI) refers to the sudden onset of kidney injury within a period of a few hours or days. Chronic kidney disease (CKD) is caused by long-term diseases such as hypertension and diabetes. Causes of acute kidney injury can be divided into The following:
- Causes that result in decreased blood flow to the kidneys (pre-renal causes), for example, hypotensive and cardiogenic shock, dehydration, and blood loss from major trauma
- Causes that result in direct damage to the kidneys (renal /intrinsic causes) such as damage to kidneys by nephrotoxic medications and other toxins, sepsis, cancers such as myeloma, autoimmune diseases or conditions that cause inflammation, or damage to the kidney tubules
- Causes that result in blockage of the urinary tract such as bladder, prostate, or cervical cancer, large kidney stones, and blood clots in the urinary tract
It is important to note that pre-renal kidney injury may progress to acute tubular necrosis (ATN) and cause intrinsic renal injury.
Urine output is a useful tool for evaluating kidney function and is used in guidelines to define AKI. Patients with AKI present with oliguria (less than 400 ml per day). The RIFLE classification (risk, injury, failure, loss of kidney function, and end-stage kidney disease) is based on serum creatinine, GFR changes, and urine output determinants. The Acute Kidney Injury Network (AKIN) classification criteria for AKI also uses serum creatinine changes and urine output; however, it does not rely on GFR changes and does not require a baseline serum creatinine.
Other laboratory investigations apart from serum creatinine play a vital role in the diagnosis of AKI and assist in differentiating between different types of acute kidney injury. This is important, as it will determine the appropriate patient management, with patients that have pre-renal causes being treated with fluid replacement. In contrast, those with renal and post-renal causes would be given fluids more conservatively.
Investigations that assist in determining if the renal injury is pre-renal, renal, or post-renal include the measurement of urine specific gravity, which is increased (greater than 1.020) in dehydration and pre-renal causes. The presence of white and red blood cells, tubular epithelial cells, casts, or crystals in the urinary sediment under light microscopy can assist in the differential diagnosis.
Fractional excretion of sodium (FeNa) is useful in distinguishing acute tubular necrosis from pre-renal uremia. It requires the measurement of serum creatinine and sodium and measurement of creatinine and sodium in spot urine specimens. Fractional excretion is calculated using the following formula:
FeNa = 100 x ( urinary sodium x serum creatinine) / (serum sodium x urinary creatinine).
A value of less than 1% indicates a pre-renal cause, and values greater than 2% indicate intrinsic causes. However, in patients receiving diuretic therapy, the FeNa is not reliable. Spot urine sodium concentrations of less than 20 mmol/l are an indicator of pre-renal AKI. Fractional excretion of urea calculated similarly to FeNa using serum urea and urine urea instead of sodium can also be used to determine the presence of pre-renal versus intrinsic AKI, with values less than 35% suggesting pre-renal injury. A urine osmolality of greater than 500 mOsm/Kg is associated with pre-renal causes, while an osmolality similar to serum (approximately 300 mOsm/kg) reflects an intrinsic cause.
Several new biomarkers have been reported to be useful for the determination of AKI and have utility in differentiation between AKI and stable CKD and pre-renal and intrinsic AKI. These include low-molecular-weight proteins, which are present in the systemic circulation and undergo glomerular filtration (for example, cystatin C, beta2-microglobulin, and retinol-binding protein) and proteins that are produced in response to cellular/tissue injury (NGAL (Neutrophil gelatinase-associated lipocalin), Kidney injury molecule 1 (KIM-1), L-type fatty acid-binding protein (L-FABP), FGF23 (Fibroblast growth factor 23), and beta-trace protein). Their optimum clinical utility will be realized with ongoing studies.