Clinical Chemistry Renal Assessment
Creatinine • Metabolic product cleared entirely by glomerular filtration • Not reabsorped • In order to see increased creatinine in serum, 50% kidney function is lost • Correlates with muscle mass • Male values higher than females
Creatinine: serum Increased Decreased Muscular dustrophy • Urinary tract obstruction • Decreased glomerular filtration • Chronic nephritis
Creatinine: Urine Increased Decreased Kidney Disease • Muscle Disease
Creatinine: Methodology • Jaffe reaction • basic reaction for creatinine • Kinetic • Principle: Protein-free filtrate(serum/urine) mixed with alkaline picrate solution forms a red “tautomer” of creatinine picrate which absorbs light at 520 nm, proportional to the amount of creatinine present • Issues • Subject to interferences from cephalosporins and alpha-keto acids • Enzymatic • New technology involving coupled reactions
Reference Range: Creatinine Serum Urine 0.8-2.0gm/ 24 hour • 0.5-1.5mg/dL
Clearance Measurements • Evaluation of renal function relies on waste product measurement, specifically the urea and creatinine • Renal failure must be severe, where only 20-20% of the nephron is functioning before concentrations of the waste products increase in the blood • The rate that creatinine and urea are cleared from the body is termed clearance
Clearance • Definition • Volume of plasma from which a measured amount of substance can be completely eliminated into urine per unit of time • Expressed in milliliters per minute • Function • Estimate the rate of glomerular filtration
Creatinine Clearance • Used to determine GFR ( glomerular filtration rate) • Most sensitive measure of kidney function • Mathematical derivation taking into effect the serum creatinine concentration to the urine creatinine concentration over a 24- hour period
Creatinine Clearance Specimen requirements Instructions for urine collection Empty bladder, discard urine, note exact time Collect, save and pool all urine produced in the next 24-hours. Exactly 24 hours from start time, empty bladder and add this sample to the collection • 24-hour urine • Keep refrigerated • Serum/Plasma • Collected during 24-hour urine collection
Creatinine clearance - Procedure • Determine creatinine level on serum/plasma - in mg/dL • Determine creatinine level on 24 hour urine • measure 24 hr. urine vol. in mL, take a aliquot • make a dilution (usually X 200) • run procedure as for serum • multiply results X dilution factor • Plug results into formula
Formula Ucr(mg/dL) X V Ur(mL/24 hour)X 1.73 P Cr(mg/dL) X 1440 minutes/ 24 hours A • U cr= urine creatinine • P cr= serum creatinine • 1.73= normalization factor for body surface area in square meters • A= actual body surface area
Nomogram Left side, find patient’s height( in feet or centimeters) On right side, find patient’s weight (lbs or kg) Using a straight edge draw a line through the points located Read the surface area in square meters, on the middle line
Reference ranges • Males • 97 mL/min- 137 mL/min • Females • 88mL/miin-128 ml/min
Creatinine Clearance Exercise • Female Patient: 5'6“ & 130 lbs. • Urine Creatinine – 98 mg/dL • Serum Creatinine – 0.9 mg/dL • 24 Hour Urine Volume – 1,200 mL • Set up calculation
Microalbumin • Important in management of diabetes mellitus • Perform an albumin/creatinine ratio
Urinalysis • In-depth renal assessment • Refer to UA notes for review of individual tests
Other Tests To Monitor Kidneys • Measurement of the non-protein nitrogen substances • BUN • Uric Acid
BUN • Blood urea nitrogen • Urea is the nitrogenous end-produce of protein / AA metabolism. • Urea is formed in the liver when ammonia (NH3) is removed and combined with CO2. • Most widely used screening test of kidney function
Blood urea nitrogen (BUN) • Serum normal values – 5.0-20.0 mg/dL • Decreased concentration seen late in pregnancy and in protein starvation. • If concentration exceeds 20.0 mg/dL, term azotemia applies. • Azotemia – nitrogen in the blood • not always kidney’s fault, excessive hemorrhage, shock, and other reasons • does not imply clinical illness, but can progress to symptomatic illness.
BUN: Methodology • Kjeldahl – a classical method for determining urea concentration by measuring the amount of nitrogen present • Berthelot reaction - Good manual method - that measures ammonia • Uses an enzyme (urease – from Jack Bean meal) to split off the ammonia • Diacetyl monoxide( or monoxime) • Popular method but not well suited for manual methods • because ➵ Uses strong acids and oxidizing chemicals
Disease correlations: BUN • Prerenal BUN( Not related to renal function ) • Low Blood Pressure ( CHF, Shock, hemorrhage, dehydration ) • Decreased blood flow to kidney = No filtration • Increased dietary protein or protein catabolism • Prerenal BUN( Not related to renal function ) • Decreased dietary protein • Increased protein synthesis ( Pregnant women , children )
Disease Correlations: BUN • Renal causes of BUN • Renal disease with decreased glomerular filtration • Glomerular nephritis • Renal failure from Diabetes Mellitus • Post renal causes of BUN ( not related to renal function ) • Obstruction of urine flow • Kidney stones • Bladder or prostate tumors • UTIs
BUN / Creatinine Ratio • Normal BUN / Creatinine ratio is 10 – 20 to 1 • Pre-renal increased BUN / Creat ratio • BUN is more susceptible to non-renal factors • Post-renal increased ratio BUN / Creat ratio • Both BUN and Creat are elevated • Renaldecreased BUN / Creat ratio • Low dietary protein or severe liver disease
Uric acid • Source • Final breakdown product of nucleic acid catabolism- from both the food we eat, and breakdown of body cells. • Uric acid is filtered by the glomerulus ( but 98 – 100 % reabsorbed ) • Increased levels • Not a primary test for kidney function - useful as a confirmatory or back - up test. • * Most useful for diagnosis and monitoring gout • Also seen during toxemia of pregnancy
Uric acid diseases • Gout • Increased plasma uric acid • Painful uric acid crystals in joints • Usually in older males ( > 30 years-old ) • Associated with alcohol consumption • Uric acid may also form kidney stones • Other causes of increased uric acid • Leukemias and lymphomas ( DNA catabolism ) • Megaloblastic anemias ( DNA catabolism ) • Renal disease ( but not very specific )
Uric Acid: Methodology 1. Phosphotungstic Acid Reduction — This is the classical chemical method for uric acid determination. In this reaction, urate reduces phosphotungstic acid to a blue phosphotungstate complex, which is measured spectrophotometrically. 2. Uricase Method — An added enzyme, uricase, catalyzes the oxidation of urate to allantoin, H2O2, and CO2. The serum urate / uric acid may be determined by measuring the absorbance before and after treatment with uricase. (Uricase breaks down uric acid.) 3. ACA — Uric acid, which absorbs light at 293 nm, is converted by uricase to allantoin, which is nonabsorbing at 293 nm. • Uric acid + 2H2O + O2 Uricase > Allantoin + H2O2 + CO2 (Absorbs at 293 nm) (Nonabsorbing at 293 nm)
Uric Acid • Normal values • Men 3.5 - 7.5 mg/dL • Women 2.5 - 6.5 mg/dL
Proteinuria Case 1 • A 20 year old patient is referred to you for ,he has been diabetic for 6 years ,he was told to have some kidney problem by his MD.He wants to know the cause of renal dysfunction. • GPE:BP 145/90 ,otherwise exam is normal • How would you proceed ? • BUN 15mg/dl, creatinine 1.0mg/dl ,U/A shows SG 1.024 ,trace protein ,a few hyaline casts • What test would you order next ? • 24h protein collection , U protein/U creatinine ratio or both?
Case 1 continued • Urine protein /Urine creatinine returns 15mg/150mg ratio(<0.1) • Does this patient have abnormal proteinuria ? • Patient wants to know if he has microalbuminuria ,you order urine micro albumin result is :60mg micro albumin /gm creatinine . • Is this abnormal, does this patient have diabetic nephropathy?
Urine Protein:Categories of persistent proteinuria • Overflow: Capacity to reabsorb normally filtered protein in proximal tubules over whelmed due to overproduction:e.g.light chains,hemoglobinuria and myoglobinuria • Tubular proteinuria: Decreased reabsorption of filtered proteins by tubules due to tubulointerstitial damage ; usually <2 gm • Glomerular proteinuria: Microalbuminuria to overt proteinuria usually>3.5 gm
Screening for Urine protein • Dipstick: Gives green color, does not check for light chains Negative – 10 mg/dl Trace – 15-25 mg/dl 1-2+ – 30-100 mg/dl 3+ – 300 mg/dl • Sulfosalicylic acid: white precipitate
Urine protein :Quantitative measurement • 24 hour collection of urine for protein normal excretion is <150 mg/24 hour • Spot urine protein/urine creatinine ratio : (as 24 h urine creatinine excretion is a function of muscle mass i.e. 15 mg/kg for females and 20mg/kg for males ) a normal ratio is 150/1500 or <0.1 . A ratio >3 indicates nephrotic range proteinuria • Case 1 has normal urine protein excretion, trace protein on u/a is due to highly concentrated urine ,pt may still have microalbuminuria
Microalbuminuria • Urine albumin excretion below detection by regular dipstick • First clinical sign of diabetic nephropathy • Incidence increases with the duration of diabetes and may be present at the diagnosis of NIDDM • Transient albuminuria may occur with fever,infection,exercise,decompensated CHF • Associated with poor glycemic control and elevated BP
Detection of Micro albuminuria: 24 hour urine collection • Normal urine protein excretion : <150mg (20% of this is albumin) • Therefore, normal urinary albumin excretion is < 30 mg/day • Microalbuminuria :urinary albumin excretion 30-300 mg/day
Microalbuminuria :Detection by Spot Urine Albumin to Urine Creatinine ratio • Easier than cumbersome 24 hr.collection • If we assume daily creatinine excretion to be 1000 mg and normal urine albumin excretion <30 mg; albumin / creatinine ratio should be less than 0.03 or 30mg/g creatinine • Thus case 1 has micro albuminuria which is likely due to diabetic nephropathy.How would you manage him now?
Why and When to Screen Patients for Microalbuminuria ? • BP control with Ace_I and ARB’s have been known to reduce microalbuminuria and delay the progression of kidney disease in diabetics • IDDM patients should be screened yearly,beginning 5 years after the onset of disease • Patients with NIDDM should be screened at presentation
Proteinuria Case 2 A70 year- old male is referred for chronic azotemia PMH: unremarkable GPE: BP120/60 , LE edema Labs: U/A SG 1.010 pH 6.0 , protein neg, glucose 2+, Uprotein /U creatinine ratio 4 BUN 30mg/dl creat.3.0, Blood Sugar 78mg/dl albumin 2.8, Hb 10 gm What other tests would you order to diagnose cause of his renal dysfunction ? UPEP,why?
Clinical Assessment of Renal Function: Glomerular Filtration Rate(GFR) • Parameters used Blood urea nitrogen Serum creatinine Endogenous creatinine clearance
Case 3 Azotemia • A 55 year old diabetic female is admitted with intractable vomiting and low urine output • Exam: BP 120/60 with postural hypotension • Labs: BUN 60, Creat. 2.0 mg/dl ( baseline 1.0mg/dl), Hb 16gm • ,U/A: SG 1.020, sediment: hyaline casts,UNa: 10 mmol/L,UOsm: 600 mosm/kg,Ucreat.150mg/dl ,Fe Na < 0.5 • Q.What is the cause of her high BUN to creatinine ratio and her renal failure? What are the other causes of high BUN to creatinine ratio
Blood Urea Nitrogen (BUN) • Catabolism of aminoacids generates NH3 NH2 2 NH3 + CO2 = C = 0 + H2O NH2 • Urea Mol wt : 60 • BUN Mol wt. : 28 • Normal BUN 10-20 mg/dl • After filtration › 50% is reabsorbed by the tubule • BUN level is related to: Renal function, protein intake, and liver function
Creatinine • Formed at a constant rate by dehydration of muscle creatine • Normally 1–2% of muscle creatine is broken into creatinine • Mol. Wt. 113 • Creatinine is freely filtered by the glomerulii and is not reabsorbed 10–15% is secreted into proximal tubule
Creatinine • Normal serum level 1–2 mg/dl • 24 hour creatinine excretion 20 mg/kg/day for males 15 mg/kg/day for females • Children, females, elderly, spinal cord injured have low serum and urine creatinine
BUN/Creatinine ratio 10:1 • Normal • Chronic renal failure
D/D in Case 3 with BUN Creatinine ratio >10:1 • Decreased perfusion • Hypovolemia • Congestive heart failure • Increased urea load • GI bleed • Glucocorticoids -Tetracycline • Hyper catabolic states • High Protein diet • Obstructive uropathy • Decreased muscle mass
Pathophysiology of Pre-renal Azotemia in Case 3 Decreased “Effective” Intravascular ADH Volume + Renal Hypoperfusion activation of RAS Diminished GFR aldosterone Low urine volume and U sodium and high Uosmolality
Case 3 :Diabetic patient continued.. • Vomiting stopped ,BP improved and BUN/creat lowered to 35/1.8mg/dl. 24 hours later she developed UTI, trimethaprim/sulfamethoxazole was started • Next day 24 hr urine output 800 mL • Exam: Unremarkable • BUN: 20 mg/dl Creat: 3.0 mg/dl • Uosm: 600 mosm/kg ,UNa: 10 mom/l, FeNa: <1% • Urine Sediment: Hyaline casts • What is the cause of < 10: 1 ,BUN to creat ratio now?
BUN/Creatinine ratio ‹ 10:1 • Decreased urea load Low protein diet Liver failure • Inhibition of creatinine secretion Cimetidine Trimethoprim Probenecid • Increased removal: Dialysis