Renal function tests

Renal function tests are very useful for effective clinical evaluation of renal failure for effective management. So it is useful for medical and allied professional students and clinical practitioners.

1. Renal function tests Dr.S.Sethupathy , M.D,Ph.D ., Professor of Biochemistry, Rajah Muthiah Medical College, Annamalai university

2. Importance Renal function tests are done for : Diagnosis of renal disease Assessment of its prognosis Monitoring renal function and damage.

3. Functions of Kidney Excretory function Excretion of endogenous waste products such as : urea , uric acid, creatinine and exogenous drugs, it metabolites, toxins etc.

4. Homeostatic function Maintenance of water balance Maintenance of electrolyte balance Maintenance of acid base balance

5. Hormonal function Kidney Synthesizes erythropoietin which stimulates erythropoiesis. Activation of 25-OH cholecalciferol to 1,25 dihydroxy cholecalciferol ( Calcitriol ) by 1-alpha hydroxylase enzyme in the kidney which promotes calcium absorption in the intestine.

6. NEPHRON Nephron is the functional unit of kidney which has two components : 1.Glomerulus 2 . Tubules Glomerulus is the filtration unit and tubules are reabsorbing unit.

7. Glomerular function Through Bowman’s capsule, an ultra-filtrate which is devoid of cells and protein formed Albumin does not pass through the membrane. So in case of glomerular damage, albumin appears in urine. Glomerular filtration depends on renal blood supply and functional state of nephrons.

8. GFR Glomerular filtration rate (GFR) is : 1. Decreased in hemorrhagic shock, anaphylactic shock due to decreased perfusion 2. In glomerulonephritis due to dysfunction of nephrons. Decrease in GFR leads to retention and accumulation of waste products such as urea, creatinine and uric acid in the plasma.

9. Tests to assess glomerular filtration Plasma creatinine: Normal level is 0.6-1.2 mg/dl. It is a better indicator of GFR as it is not influenced by diet or protein catabolism as in case of urea. It is inversely related to GFR. Plasma urea : Normal plasma urea level is 15-40 mg/dl. Plasma urea nitrogen is 7-18 mg/dl .

10. PAH clearance to assess renal plasma flow PAH is filtered at the glomerulus as well as secreted by the tubules Completely removed by the kidney by single passage of blood through the kidney R enal perfusion flow(RBF)= U pah / p pah x V V= volume of urine (ml/min) normal – 600 ml min

11. Renal blood flow

12. Blood urea It is also an index of GFR. Pre renal failure - dehydration. urea is increased more than creatinine because it is reabsorbed by the tubular cells. In renal failure, both will increase . Post renal uremia-both will increase

13. Creatinine clearance Clearance is defined as the volume of blood or plasma in ml that is completely cleared of a substance (in case of creatinine clearance creatinine is cleared) per unit time and is expressed as ml per min.

14. GFR Creatinine clearance = UV/P U=concentration of creatinine in urine (mg/dl) P=concentration of creatinine in plasma (mg/dl) V- volume of urine passed per minute *ml/min). 24 hours urine is collected and the total volume is divided by 1440 .

15. GFR Inulin clearance : It is the reference method for determination of GFR. It is not routinely done because it is given exogenously and the maintenance of its blood level requires monitoring. Urea clearance It is lower than inulin clearance because urea is reabsorbed in the tubules.

16. Ideal GFR marker Marker should be freely filtered It should be neither secreted nor reabsorbed and it’s blood level should be constant. Creatinine being an endogenous substance its level is maintained, freely filtered but a small amount is secreted by the tubules (10% of creatinine excreted is tubular component).

17. Plasma cystatin C Normal level is 0.5-1.0 mg/L. Cystatin C is produced at a constant rate and is freely filtered by glomerulus. It is completely reabsorbed by the tubules and degraded in the tubules. The blood level is not dependent upon age, sex, muscle mass or inflammatory processes. It is a better indicator of GFR than creatinine Extremely sensitive to minor changes in GFR during the course of chronic kidney disease.

18. Interpretation Normal value: male: 75-125 ml/min Female: 65-115 ml/min Decreased creatinine clearance is a very sensitive indicator of reduced glomerular filtration rate (GFR ). In old age, the clearance is decreased . It is helpful in the early detection of functional impairment of kidney and also for monitoring the patients with renal insufficiency.

19. The Crockcroft-Gault equation GFR mL /minute = [(140 – age in years) × (wt, kg)]/ 72 × P (mg/L) × 0.85 (if subject is female). If converting to SI units (GFR in micromoles per liter) is desired, replace 72 in the denominator with 0.84.

20. Modification of Diet in Renal Disease  (MDRD) FORMULA GFR ( mL /minute/(1.73 m2)) = 186 × (serum creatinine [mg/ dL ]–1.154) × (age in years)-0.203 × (0.742 if female) × (1.210 if African American). If converting to SI units (GFR in micromoles per liter) is desired, replace 186 with 32,788 .

21. MDRD – clinical use Estimates GFR adjusted for body surface area. Designed for use with laboratory creatinine test. Is more accurate than creatinine clearance measured from 24-hour urine collections or estimated by the Cockcroft- Gault formula.

22. Test to assess glomerular integrity The glomerular membrane is impermeable to albumin. Low molecular proteins are filtered reabsorbed and catabolized by tubular cells. Albumin excretion is less than 30 mg/24 hours. Urine test for albumin is negative in normal subjects.

23. Proteinuria It may be due to 1.Increase in filtered load due to glomerular damage and vascular permeability. It is called glomerular proteinuria. 2.Increased concentration of low molecular weight proteins in circulation resulting in Overflow proteinuria.   3.Decrease in the tubular reabsorption of proteins resulting in tubular proteinuria .

24. Glomerular proteinuria Early morning urine specimen or 24hrs urine specimen tested for albumin . Benign proteinuria- 300mg/day Pathological proteinuria- 300-1000 mg/day Glomerular proteinuria - > 1 gm /day. In nephrotic syndrome massive proteinuria is seen >3 gm/day . Diabetic nephropathy , Chronic glomerulonephritis , Hypertension .

25. Micro albuminuria or Minimal albuminuria or Pauci albuminuria If albumin is detected in a urine sample collected at random, over 4 hours, or overnight, the test may be repeated and/or confirmed with urine that is collected over a 24-hour period (24-hour urine). The quantity of albumin in urine is 30-300 mg/day in micro albuminuria. It is an early indication of nephropathy in diabetic patients and hypertensive patients. It is also expressed as albumin creatinine ratio Albumin creatinine ratio – 30-300mg albumin /gm of creatinine -0.03- 0.3

26. Over flow proteinuria Hemoglobinuria in hemolytic diseases Myoglobinuria in muscle crush injury Bence - Jones protein in multiple myeloma Tubular proteinuria Due to decreased functional nephrons the remaining nephrons over-work . Tubular reabsorption of proteins is impaired causing tubular proteinuria. Proximal renal tubular damage -increased excretion of β- microglobulin .

27. Tests of tubular function Urine specific gravity Normal value is 1.016-1.022 Fixed specific gravity at 1.010 is seen in chronic kidney disease due to tubular dysfunction. Specific gravity is increased in diabetes mellitus, adrenal insufficiency Specific gravity is decreased in diabetes insipidus (ADH insufficiency).

28. Urine osmolality Normal range is 400-850 mOsm /kg Plasma osmolality is 285-295 mOsm /kg. Normally the urine- plasma osmolality ratio is above 1.3. Urine osmolality - decreased in diabetes insipidus and the ratio is lesser than 1 . Concentrating ability of distal tubules and collecting ducts decreased. Due to renal defect ( nephrogenic diabetes insipidus which do not response to ADH ) Central diabetes insipidus due to ADH deficiency.

29. Osmolality

30. Water deprivation test (Urine concentration test) For the test, patient should stop taking any fluid for 8 hours (C aution - dehydration ). In normal subjects urine osmolality will be more than 800 mOsm /kg, plasma osmolality should not exceed 295 mOsm /kg and the ratio will be more than 2. In diabetes insipidus , the ratio is below 1 (0.2-0.7). water deprivation with ADH stimulation, urine osmolality - more than 800mOsm/kg - central D.Insipidus but in nephrogenic diabetes insipidus,less than 300mOsm/kg.

31. Urine dilution test The patient is not allowed to drink any fluid after mid night. Bladder is emptied at 7 am and water load (1200 ml over the next 30 min) is given. Hourly urine samples are collected for next four hours. The specific gravity of at least one sample should fall to 1.003 and osmolality to 50 mOsm /kg. This test is more sensitive and less harmful than concentration test.

32. Urine acidification test Enteric coated capsules containing ammonium chloride at a dose of 0.1 g / kg body wt is given . In the liver, NH 3 is converted to urea and HCl is produced which is excreted by kidney. Urine is collected hourly from 2 to 8 hours after ingestion. At least one sample should have a pH of 5.3 . In type I distal renal tubular acidosis, urinary pH rarely falls below 6 and never falls below 5.3.

33. Phenolsulfonthalein (PSP ) test Phenolsulfonthalein test dye 6 mg in 1 ml saline is given intravenously and urine samples are collected at 15, 30, 60, 120 minutes. If 15 minute urine contains 25% or more, the test is normal. If it is less than 23%, it indicates impaired renal excretory function. Normally 40-60 % in 1 hour and 20-25% in the second hour excreted.

34. Volume of urine More than 2500 ml / 24 hours it is called polyuria. It can be due to : 1. Increase in water loss due to either diminished tubular dysfunction with decreased concentration ability or Anti diuretic hormone deficiency. In ADH deficiency ( diabetes insipidus ), Urine specific gravity will be lowered 2. Due to excessive solute loss and osmotic diuresis - In case of diabetic mellitus due to glucosuria , there will be polyuria. In this case urine specific gravity will be increased

35. Color of the urine Normally it is pale yellow or amber color. Hematuria or hemoglobinuria produce a dark brown color. By microscopy, hematuria can be detected by the presence of intact red blood cells. It can be due to stone, tumor, injury, infection .

36. pH of urine usually acidic- pH 6 (4.5-8-pH ) Specific gravity normally varies from 1.016 to 1.032 Osmolality On average fluid intake, 300-900 mOsm /kg Odor foul smell indicates bacterial infection

37. When urine output is lesser then 400 ml / 24 hours, it is called oliguria. If no urine is passed, then, it is called anuria. Oliguria, anuria can be the result of following: Diminished perfusion of kidney ( cardiac failure, Hypotension, shock ), Renal disease such as acute glomerulonephritis, tubular necrosis, Obstruction to the outflow e.g. bilateral tumor in the bladder, renal stones, prostate enlargement etc.

38. Urine investigations Protein , Blood , Sugar, pH , specific gravity, osmolality, Blood investigations   Blood urea – 15 – 40 mg/dl Serum creatinine – 0.6 – 1.2 mg/dl Serum uric acid - 4- 7 mg/dl in men , 3-6 mg/ dL in women

39. Serum sodium - 135- 145 mmol /l Serum potassium – 3.5 – 5 mmol /l Serum chloride- 96 – 106 mmol /l Serum bicarbonate - 23 – 27 mmol /l Arterial blood pH - 7.35 – 7.45 paCO2 - 35 – 45 mm of Hg paO2 - 80-100 mm of Hg 

40. Fractional excretion of HCO3 %

41. Fractional excretion of PO4

42. Phosphate excretion A daily  phosphate excretion  of <3.2 mmol (100 mg) and a  fractional excretion of phosphate  <5% ( normal value  is 15–20%) allow diagnosis of non-renal  phosphate  loss. A urinary  phosphate excretion  >3.2 mmol (100 mg) or a  fractional excretion  >5% is indicative of renal phosphate wasting. Fractional excretion of phosphate ( FeP ) is associated with end-stage renal disease patients with CKD 3b and 5 FGF23 ( Phosphotonin )is secreted from bones and acts on the kidneys to induce phosphaturia and suppress active vitamin D synthesis It maintain  phosphate  homeostasis.

43. Phosphate excretion Decreased in X-linked hypophosphatemic rickets Osteogenic osteomalaia - (failure of inactivation of phosphatonin ) Hyperparathyroidism Increased in hypoparathyroidism Reduced phosphate reabsorption in hypercalciuric stone, renal tubular dysfunction

44. Beta 2 microglobulin Metabolized in tubules Urinary measurement of beta 2 microglobulin provide a sensitive index of assessing tubular integrity beta 2   microglobulin  in  urine is 0-0.3 µg/ mL. In  serum  or plasma samples is 0-3 µg/ mL.  

45. Fractional excretion of Na Fractional excretion of Na = Urine Na / serum Na x 100 Urine creatinine / Sr.creatinine < 1% in prerenal azotemia > 1.5% in acute tubular necrosis > 3% in postrenal failure

46. Renal failure index RFI ( mmol /L) = Urine Na x serum creatinine / urine creatinine Prerenal failure < 1 mmol /L Post renal failure and nephrotoxic renal failure > 3 mmol /L

47. Renal Panel Urea , creatinine , uric acid , creatinine clearance, eGFR , cystatin Sodium, potassium, bicarbonate, chloride Calcium, phosphorus Plasma glucose Serum albumin ,total protein CBC and Urinalysis Urine albumin creatinine ratio Beta 2microglobulin Sr. AST, ALT ALP, bilirubin

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