Overview Of Renal Function Tests

The production of hormones like erythropoietin, 1,25-dihydroxyvitamin D, and renin, as well as the excretion of waste products and toxins like urea, creatinine, and uric acid, as well as the regulation of extracellular fluid volume, serum osmolality, and electrolyte concentrations, all depend on the kidneys. The nephron, which includes the glomerulus, proximal and distal tubules, and collecting duct, is the functional unit of the kidney. In the treatment of patients with kidney disease or other illnesses that impact renal function, assessment of renal function is crucial. Renal function tests are helpful in detecting the existence of renal illness, tracking the kidneys' healing progress, and figuring out how the disease is progressing. According to the National Institutes of Health, the overall prevalence of chronic kidney disease (CKD) is approximately 14%. Worldwide, the most common causes of CKD are hypertension and diabetes.

The technique or test required will determine the necessary specimen collection. In most cases, a random blood sample is sufficient to determine serum creatinine and blood urea nitrogen (BUN) values; no extra patient preparation is necessary. The consequence of recent high protein diet, however, may significantly raise blood creatinine and urea levels. Additionally, the level of hydration can significantly affect how BUN is measured.It is crucial that urine be precisely collected during the requisite timeframe for scheduled urine collections, such as the 24-hour urine creatinine clearance, since under- or over-collection will have an impact on the findings. Therefore, a timed collection of 5 to 8 hours is preferred over a collection of 24 hours. Midstream urine must be collected for examination since epithelial cells and commensal bacteria are less likely to contaminate this sample.

Assessment of Renal Function

A variety of clinical laboratory tests can be performed to look into and assess kidney function. Estimating the glomerular filtration rate (GFR) and looking for proteinuria are the clinical tests that are most useful for determining how well the kidneys are functioning (albuminuria).

Glomerular Filtration Rate

Glomerular filtration rate is the most accurate overall measure of glomerular function (GFR). GFR, or the clearance of a material from the blood, is the rate in milliliters per minute at which compounds in plasma are filtered by the glomerulus. An adult male's GFR should be between 90 and 120 mL per minute. The following traits make for a perfect GFR marker:

 It should continuously manifest endogenously in the plasma.

At the glomerulus, it should go via a free filter.

The renal tubule is unable to re-absorb or emit it.

It shouldn't be eliminated using extrarenal means.

Exogenous indicators of GFR are utilized because there is presently no corresponding endogenous marker. The standard approach for estimating GFR is to use the polysaccharide inulin for the assessment of GFR. In order to ascertain the rate of inulin clearance, blood levels must be measured following the administration of inulin for a certain amount of time. Other exogenous markers include radioisotopes like technetium-99-labeled diethylene-triamine-pentaacetate and chromium-51 ethylene-diamine-tetra-acetic acid (51 Cr-EDTA) (99 Tc-DTPA). The non-radioactive contrast agent, iohexol, is the most promising exogenous marker, especially in children. The inconvenience of using exogenous markers, specifically the need for testing to be done in specialized facilities and the challenge of assaying these substances, has encouraged the use of endogenous markers.

Creatinine

Creatinine is the endogenous marker that is most frequently used to evaluate glomerular function. An indication of GFR is provided by the computed creatinine clearance. Since 24-hour collections are notoriously inaccurate, it is necessary to collect urine over a period of 24 hours, or better over a precisely scheduled period of 5 to 8 hours. The equation is then used to compute creatinine clearance:

C = (U x V) / P

C = clearance, U = urinary concentration, V = urinary flow rate (volume/time i.e. ml/min), and P = plasma concentration

Correcting creatinine clearance for body surface area is necessary. One of the main problems impacting the accuracy of this test is improper or partial urine collection, hence timed collection is helpful. Furthermore, creatinine overestimates GFR by 10% to 20% as a result of tubular secretion.

The body continuously produces creatinine, a byproduct of the breakdown of creatine phosphate in muscle. Most of the time, the kidney is the only organ that removes creatinine from the blood. Increased blood creatinine is the result of decreased renal clearance. The daily production of creatinine is influenced by muscle mass. As a result, males and females with lower creatinine readings in children and those with less muscle mass have different creatinine ranges. Creatinine levels are also influenced by diet. Red meat consumption can cause a 30% change in creatinine. Lower creatinine readings are observed in pregnancy when GFR rises. Additionally, serum creatinine is a later indicator of renal impairment-renal function is decreased by 50% before a rise in serum creatinine is observed.

The Modified Diet in Renal Disease (MDRD) and the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equations both use serum creatinine to estimate GFR. Since these eGFR equations take into account factors including race, age, and gender, they are preferable than serum creatinine alone. According to the phases of renal disease, GFR is divided.

Stages of chronic kidney disease (CKD) according to Kidney Disease Improving Global Outcomes (KDIGO):

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)

A nitrogen-containing substance called urea, sometimes known as BUN, is produced in the liver as a byproduct of protein metabolism in the urea cycle. The kidneys remove around 85% of the urea; the remainder is expelled through the GI tract. When renal clearance declines (as in acute and chronic renal failure/impairment), serum urea levels rise. Other disorders include upper GI hemorrhage, dehydration, catabolic states, and high protein diets that are unrelated to renal illnesses can also cause a rise in urea. In famine, a low-protein diet, and severe liver illness, urea production may be reduced. Although urea is elevated sooner in renal disease, serum creatinine is a more precise measure of renal function.

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

All nucleated cells in the body generate cystatin C, a low-molecular-weight protein that serves as a protease inhibitor. It continuously forms and is readily filtered by the kidneys. The glomerular filtration rate and serum levels of cystatin C are negatively linked (GFR). In other words, analogous to creatinine, high numbers represent low GFRs and low values represent greater GFRs. Cystatin C is handled differently by the kidneys than creatinine is. Contrary to creatinine, which is freely filtered by both glomeruli, cystatin C is reabsorbed and processed by proximal renal tubules after it has been filtered. Thus, under typical circumstances, cystatin C does not significantly enter the final voided urine. 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

The abnormal presence of albumin in the urine is referred to as albuminuria. Since there is no such biological molecule as microalbumin, the word is presently exclusively used to refer to urine albumin. In diabetics, albuminuria is a sign for the early stages of nephropathy. It is a marker for chronic renal impairment as well as an independent indicator of cardiovascular disease since it denotes increased endothelial permeability. Urine albumin can be assessed as an albumin/creatinine ratio in 24-hour urine collections or early morning/random sample. When a urinary tract infection is ruled out and albuminuria is present on two separate occasions, glomerular dysfunction is the likely cause. Albuminuria for three months or longer is a sign of chronic renal 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:

 1.Glomerular proteinuria: Caused by defects in permselectivity of the glomerular filtration barrier to plasma proteins (for example, glomerulonephritis or nephrotic syndrome)

2.Tubular proteinuria: Caused by incomplete tubular reabsorption of proteins (for example, interstitial nephritis)

3.Overflow proteinuria: Caused by increased plasma concentration of proteins (for example, multiple myeloma-Bence Jones protein, myoglobinuria)

4.Urinary tract inflammation or tumor

5.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).

Tests of Tubular Function

The reabsorption of electrolytes, water, and preserving the acid-base balance are all crucial functions of the renal tubules. Urine may be used to test the electrolytes sodium, potassium, chloride, magnesium, phosphate, and glucose. Evaluation of the urine tubules' capacity to concentrate allows for measurement of urine osmolality. Urinary osmolality more than 750 mOsmol/Kg H2O denotes normal tubular concentration capacity. In order to rule out nephrogenic diabetes insipidus, a water deprivation test might be employed. The diagnosis of distal renal tubular acidosis (RTA) with inability to acidify the urine to a pH of less than 5.3 can also be confirmed by an ammonium chloride test. Aminoaciduria, glycosuria, phosphaturia, and bicarbonate wasting are all symptoms of Fanconi's syndrome (proximal RTA).

Urine Analysis

Analyzing urine characteristics is a step in the illness diagnostic process. Physical, chemical, and microscopic inspection are all part of it. During the physical examination, color and clarity are evaluated. Dehydration causes urine to become darker in color than it would normally be, which is straw-colored. Red urine may be a sign of porphyria or hematuria, as well as the use of foods like beets. When a urinary tract infection is present and pyuria is present, cloudy urine may be visible. Refractometry or a urine dipstick can be used to determine specific gravity, which is a sign of the kidneys' capacity for concentration. Specific gravity should fall between the physiological range of 1.003 to 1.030. Concentrated pee has a higher specific gravity, while diluted urine has a lower specific gravity.Utilizing chemical analysis, urine dipsticks give qualitative analyses of several analytes in urine. The presence of protein, glucose, blood, ketones, bilirubin, urobilinogen, nitrite, and leukocyte esterase may all be found using dry chemical techniques. You can use the dipstick as a point-of-care test. To interpret the results, the color changes that occur when the urine interacts with the chemical reagents impregnated on the dipstick paper are compared to the color chart reference.

In healthy urine samples, analytes evaluated on urine dipstick-protein should not be visible. Normal urine does not include bilirubin. When the renal threshold of 180 mg/dl is lowered, glucose can be noticed in diabetes mellitus, pregnancy, and renal glycosuria but not in healthy persons. Results might be impacted by the presence of certain antibiotics and ascorbic acid (vitamin C). After an infection or damage to the renal system, blood may be present, and ascorbic acid will cause a false negative. The globin part of hemoglobin is what the pee dipstick detects, therefore it cannot tell if there is myoglobin or hemoglobin present in the urine.Additionally, hemoglobinuria and intact red blood cells (RBC) are seen. RBCs that are normal show the presence of "blood" on the pee dipstick test, which distinguishes rhabdomyolysis from hematuria, when RBCs are also seen. White blood cells (WBC) and RBC per high-power field in normal urine range from 0 to 3 and 0 to 5, respectively. Fasting, violent vomiting, and diabetic ketoacidosis all contain ketones. Only acetoacetate and acetone can be found in urine, not the ketone beta-hydroxybutyrate. Conjugated hyperbilirubinemia results in the detection of bilirubin. Although urobilinogen is occasionally seen, it is missing in conjugated hyperbilirubinemia and is more common when prehepatic jaundice and hemolysis are present. Leucocyte esterase and nitrite are markers for urinary tract infections.

Acute versus Chronic Renal Impairment

The abrupt development of kidney injury over the course of a few hours or days is referred to as acute renal impairment or acute kidney injury (AKI). Diabetes and hypertension are two long-term illnesses that contribute to chronic kidney disease (CKD). These are the categories of acute renal damage causes:

1.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

2.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

3.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

AKI is defined by recommendations that use urine output as a helpful indicator of kidney health. Oliguria is a presenting sign in AKI patients (less than 400 ml per day). Serum creatinine, changes in GFR, and indicators of urine output form the basis of the RIFLE classification, which stands for risk, injury, failure, loss of kidney function, and end-stage renal disease. Serum creatinine fluctuations and urine output are both used in the Acute Kidney Injury Network (AKIN) categorization criteria for AKI; however, GFR changes and a baseline serum creatinine are not taken into account.

In addition to serum creatinine, several laboratory tests are crucial for the diagnosis of AKI and help distinguish between various forms of acute renal damage. 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.Measuring urine specific gravity, which is elevated (more than 1.020) in dehydration and pre-renal causes, is one investigation that helps identify if the renal damage is pre-renal, renal, or post-renal. When viewing the urine sediment under a light microscope, the presence of white and red blood cells, tubular epithelial cells, casts, or crystals might help narrow down the differential diagnosis.

It helps to distinguish between pre-renal uremia and acute tubular necrosis using fractional sodium excretion (FeNa). It calls for the analysis of spot urine samples and the detection of salt and creatinine in serum. This formula is used to determine fractional excretion:

FeNa = 100  x ( urinary sodium x serum creatinine) / (serum sodium x urinary creatinine).

Pre-renal causes are indicated by values lower than 1%, and intrinsic causes are indicated by values higher than 2%. The FeNa is unreliable in individuals who are on diuretic treatment, though. Pre-renal AKI is indicated by sodium concentrations in the urine spots that are less than 20 mmol/l. To distinguish between pre-renal versus intrinsic AKI, fractional excretion of urea may be determined similarly to FeNa using serum and urine urea instead of sodium, with values less than 35% indicating pre-renal damage. An osmolality similar to serum (about 300 mOsm/kg) indicates an intrinsic cause, but urine with an osmolality over 500 mOsm/kg is linked to pre-renal causes.