Assessment of kidney function occurs in different ways, using the presence ofsymptoms andsigns, as well as measurements using urine tests, blood tests, and medical imaging.
Functions of a healthy kidney include maintaining a person'sfluid balance, maintaining anacid-base balance; regulating electrolytessodium, and otherelectrolytes;clearingtoxins; regulatingblood pressure; and regulatinghormones, such aserythropoietin; and activation ofvitamin D. The kidney is also involved in maintaining blood pH balance.
The functions of the kidney include maintenance ofacid-base balance; regulation offluid balance; regulation ofsodium,potassium, and otherelectrolytes;clearance oftoxins; absorption ofglucose,amino acids, and other small molecules; regulation ofblood pressure; production of varioushormones, such aserythropoietin; and activation ofvitamin D.
TheGlomerular filtration rate (GFR) is regarded as the best overall measure of the kidney's ability to carry out these numerous functions. An estimate of the GFR is used clinically to determine the degree of kidney impairment and to track the progression of the disease. The GFR, however, does not reveal the source of the kidney disease. This is accomplished by urinalysis, measurement of urine protein excretion, kidney imaging, and, if necessary, kidney biopsy.[1]
Much of renal physiology is studied at the level of thenephron – the smallest functional unit of the kidney. Each nephron begins with afiltration component that filters theblood entering the kidney. This filtrate then flows along the length of the nephron, which is a tubular structure lined by a single layer of specializedcells and surrounded bycapillaries. The major functions of these lining cells are thereabsorption of water and small molecules from the filtrate into the blood, and thesecretion of wastes from the blood into theurine.[citation needed]
Proper function of the kidney requires that it receives and adequately filters blood. This is performed at the microscopic level by many hundreds of thousands of filtration units calledrenal corpuscles, each of which is composed of aglomerulus and aBowman's capsule. A global assessment of renal function is often ascertained by estimating the rate of filtration, called theglomerular filtration rate (GFR).
Clinical assessment can be used to assess the function of the kidneys. This is because a person with abnormally functioning kidneys may have symptoms that develop. For example, a person with chronic kidney disease may developoedema due to failure of the kidneys to regulate water balance. They may develop evidence of chronic kidney disease, that can be used to assess its severity, for examplehigh blood pressure,osteoporosis oranaemia. If the kidneys are unable to excreteurea, a person may develop a widespreaditch orconfusion.[citation needed]
Part of the assessment of kidney function includes the measurement of urine and its contents. Abnormal kidney function may cause too much or too little urine to be produced. The ability of the kidneys to filter protein is often measured, asurine albumin orurine protein levels,[2] measured either at a single instance or, because of variation throughout the day, as 24-hour urine tests.[citation needed]
Blood tests are also used to assess kidney function. These include tests that are intended to directly measure the function of the kidneys, as well as tests that assess the function of the kidneys by looking for evidence of problems associated with abnormal function. One of the measures of kidney function is the glomerular filtration rate (GFR). Other tests that can assess the function of the kidneys include assessment ofelectrolyte levels such aspotassium andphosphate, assessment of acid-base status by the measurement ofbicarbonate levels from a vein, and assessment of thefull blood count foranaemia.[citation needed]
The glomerular filtration rate (GFR) describes the volume of fluid filtered from therenal (kidney)glomerular capillaries into theBowman's capsule per unit time.[3] Creatinine clearance (CCr) is the volume ofblood plasma that is cleared ofcreatinine per unit time and is a useful measure for approximating the GFR. Creatinine clearance exceeds GFR due to creatinine secretion,[4] which can be blocked bycimetidine. Both GFR and CCr may be accurately calculated by comparative measurements of substances in the blood and urine, or estimated by formulas using just a blood test result (eGFR andeCCr) The results of these tests are used to assess the excretory function of the kidneys.Staging ofchronic kidney disease is based on categories of GFR as well asalbuminuria and cause ofkidney disease.[2]
Central to the physiologic maintenance of GFR is the differential basal tone of the afferent and efferent arterioles (see diagram). In other words, the filtration rate is dependent on the difference between the higher blood pressure created by vasoconstriction of the input or afferent arteriole versus the lower blood pressure created by lesser vasoconstriction of the output or efferent arteriole.
GFR is equal to therenal clearance ratio when any solute is freely filtered and is neither reabsorbed nor secreted by the kidneys. The rate therefore measured is the quantity of the substance in the urine that originated from a calculable volume of blood. Relating this principle to the below equation – for the substance used, the product of urine concentration and urine flow equals the mass of substance excreted during the time that urine has been collected. This mass equals the mass filtered at the glomerulus as nothing is added or removed in the nephron. Dividing this mass by the plasma concentration gives the volume of plasma which the mass must have originally come from, and thus the volume of plasma fluid that has entered Bowman's capsule within the aforementioned period of time. The GFR is typically recorded in units ofvolume per time, e.g., milliliters per minute (mL/min). Compare tofiltration fraction.[citation needed]
There are several different techniques used to calculate or estimate the glomerular filtration rate (GFR or eGFR). The above formula only applies for GFR calculation when it is equal to the clearance rate.
The normal range of GFR, adjusted forbody surface area, is 100–130 average 125 (mL/min)/(1.73 m2) in men and 90–120 (mL/min)/(1.73 m2) in women younger than the age of 40. In children, GFR measured by inulin clearance is 110 (mL/min)/(1.73 m2) until 2 years of age in both sexes, and then it progressively decreases. After age 40, GFR decreases progressively with age, by 0.4–1.2 mL/min per year.[citation needed]
Estimated GFR (eGFR) is now recommended by clinical practice guidelines and regulatory agencies for routine evaluation of GFR whereas measured GFR (mGFR) is recommended as a confirmatory test when more accurate assessment is required.[5]
The kidney function can also be assessed withmedical imaging. Some forms of imaging, such askidney ultrasound orCT scans, may assess kidney function by indicating chronic disease that can impact function, by showing a small or shrivelled kidney.[citation needed]. Other tests, such asnuclear medicine tests, directly assess the function of the kidney by measuring the perfusion and excretion of radioactive substances through the kidneys.[citation needed]
A decreased renal function can be caused by many types ofkidney disease. Upon presentation of decreased renal function, it is recommended to perform ahistory andphysical examination, as well as performing arenal ultrasound and aurinalysis.[citation needed] The most relevant items in the history aremedications,edema,nocturia, grosshematuria,family history of kidney disease,diabetes andpolyuria. The most important items in a physical examination are signs ofvasculitis,lupus erythematosus,diabetes,endocarditis andhypertension.[citation needed]
A urinalysis is helpful even when not showing any pathology, as this finding suggests an extrarenal etiology.Proteinuria and/orurinary sediment usually indicates the presence ofglomerular disease.Hematuria may be caused by glomerular disease or by a disease along theurinary tract.[citation needed]
The most relevant assessments in arenal ultrasound are renal sizes,echogenicity and any signs ofhydronephrosis. Renal enlargement usually indicates diabetic nephropathy,focal segmental glomerular sclerosis ormyeloma. Renal atrophy suggests longstanding chronic renal disease.[citation needed]
Risk factors for kidney disease include diabetes, high blood pressure, family history, older age, ethnic group and smoking.For most patients, a GFR over 60 (mL/min)/(1.73 m2) is adequate. But significant decline of the GFR from a previous test result can be an early indicator of kidney disease requiring medical intervention. The sooner kidney dysfunction is diagnosed and treated the greater odds of preserving remaining nephrons, and preventing the need for dialysis.
| CKD stage | GFR level((mL/min)/(1.73 m2)) |
|---|---|
| Stage 1 | ≥ 90 |
| Stage 2 | 60–89 |
| Stage 3 | 30–59 |
| Stage 4 | 15–29 |
| Stage 5 | < 15 |
Theseverity of chronic kidney disease (CKD) is described by six stages; the most severe three are defined by the MDRD-eGFR value, and first three also depend on whether there is other evidence of kidney disease (e.g.,proteinuria):
Note: others add a "T" to patients who have had a transplant regardless of stage.
Not all clinicians agree with the above classification, suggesting that it may mislabel patients with mildly reduced kidney function, especially the elderly, as having a disease.[6][7] A conference was held in 2009 regarding these controversies by Kidney Disease: Improving Global Outcomes (KDIGO) on CKD: Definition, Classification and Prognosis, gathering data on CKD prognosis to refine the definition and staging of CKD.[8]