Lecture 4: Clinical Evaluation of Renal Function Flashcards
what is clearance?
the volume of plasma cleared entirely of a substance in a unit of time, represented by UV/P
how can you ideally estimate the amount of plasma filtered by the glomeruli?
inulin (assume that the amount produced is the amount excreated in steady state
Excreted amount of inulin = Urine inulin concentration x Urine volume
however not used in clinical practice
Why is inulin the gold standard for GFR assessment
Totally filtered by glomerulus
Not secreted or reabsorbed by tubules
Not in any way altered by tubules
The amount excreted = The amount filtered.
What is used in clinical practice to assess GFR? and why?
Creatinine Is:
endogenously produced from the metabolism of creatine in skeletal muscle
Excreted by the kidneys
Freely filtered across the glomerulus and is neither reabsorbed nor metabolized by the kidney
GFR = CrCl = UcrV/Pcr
Excreted amount of creatinine= Urine creatinine x Urine volume
What are limitations to the use of creatinine clearance
The clearance is slightly greater than the GFR because the excreted amount exceeds the amount filtered as a result of some tubular secretion of creatinine
Incomplete urine Collection
What does serum creatinine tell us?
Creatinine excretion is constant, serum creatinin is inversely related to GFR
the higher the serum creatinine, the worse the kidney function
what affects serum creatinine?
muscle mass, diet, creatine supplements, malnutrition, and amputations.
Cockroft – Gault formula- creatinine clearance
[(140-age) x lean body weight in kg] / (72 x serum creatinine)
Multiply x 0.85 for women
MDRD formula use
MDRD formula not validated in children, pregnant women, certain ethnic groups, those with unusual muscle mass, body habitus and weight
MDRD formula tends to underestimate GFR for “near-normal” creatinine
Reported by labs as >60 ml/min
Cr measurement in labs needs to be standardized
Describe the use of serum creatinin to measure kidney function in CKD
A good measure of kidney function but must be interpreted in light of muscle mass
24 h urine collection to determine CrCl
Cockroft- Gault or MDRD formula
What can ultrasound be used for in assessing renal disease?
Assess Kidney size
Morphology
Rule out obstruction
What can CT be used for in assessing renal disease?
Provides complimentary information to that obtained
Ultrasound: for example to further characterize cysts
Diagnosis of kidney stones
What occurs in acute kidney injury?
GFR is falling rapidly
Creatinine excretion decreases
Serum creatinine and BUN rise each day
Creatinine is changing so the patient is not in a steady state.
Therefore, cannot use creatinine clearance but can follow the rise in BUN and creatinine.
what is the clinical use of urine pH?
low urine pH may be associated with uric acid stones
calcium phosphate stones and UTIs with urea splitting organisms may result in high pH
what can proteinuria tell us clinically?
abnormal basement membrane, leakage from tubules (Balkan nephropathy, tubulointerstitial nephritis) overflow (multiple myeloma - Bence Jones proteins)
what an RBCs in urine tell us?
with proteinuria, possible intrinsic renal problem
what can glucose in urine tell us?
renal disease
what can ketones in urine tell us?
diabetic ketoacidosis, states of starvation
What can urobilirubinogen tell us?
if obstructive jaundice –> urobilirubinogen decreased
if not obstructive –> urinary urobilirubinogen increased
What can nitrite in urine tell us?
screen test for gram negative bacteria
ascorbate may produce false negative
What can leukocytes in urine tell us?
glycosuria, high specific gravity, cephalexin, tetraycycline therapy
what is a bland sedimentt?
microscopic examination of normal urine. may see hyaline casts (increase with fever and exercise) Patients with prerenal azotemia or obstruction may also have some formed elements
what is acute tubular necrosis sediment?
most common cause of acute renal failure, classically has tubular cells, granular debris, pigmented granular casts
what is nephritic sediment?
it has red blood cells, often dysmorphic and acanthocytes (protein on dipstick) with granular and RBC casts indicating hematuria is glomerular in origin. Present in glomerulonephritis and renal vasculitis.
what is nephrotic sediment?
4+ protein, fatty casts and oval fat bodies. Seen in patients with glomerulonephritis with nephrotic range proteinuria. non-proliferative glomerulonephritis with heavy proteinuria will have heavy nephrotic sediment.
what does urine in pyelonephritis and acute interstitial nephritis look like?
many WBCs and white blood cell casts. culture will be positive in pyelonephritis but not in acute interstitial nephritis.
where do you see broad casts?
chronic renal failure in which tubules of functioning nephrons have dilated
what are telescoped urine?
it has elements of chronicits (broad casts) and more acute disease (granular casts and RBC casts). Characteristic but not specific for rapidly progressive glomerulonephritis
Crystals found in urine
calcium oxalate - acid urine, increased in ethylene glycol overdose
uric acid crystals - acid urine
cystine crystals - hexagons, pathologic, acid urine, confirmed by nitroprusside test
triple phospage crystals - alkaline urine, urea splitting bacteria and infection, look like coffins
calcium carbonate crystlas - alkaline urine
