Lecture 4: Clinical Evaluation of Renal Function

Question 1

what is clearance?

Answer 1

the volume of plasma cleared entirely of a substance in a unit of time, represented by UV/P

Question 2

how can you ideally estimate the amount of plasma filtered by the glomeruli?

Answer 2

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

Question 3

Why is inulin the gold standard for GFR assessment

Answer 3

Totally filtered by glomerulus
Not secreted or reabsorbed by tubules
Not in any way altered by tubules
The amount excreted = The amount filtered.

Question 4

What is used in clinical practice to assess GFR? and why?

Answer 4

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

Question 5

What are limitations to the use of creatinine clearance

Answer 5

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

Question 6

What does serum creatinine tell us?

Answer 6

Creatinine excretion is constant, serum creatinin is inversely related to GFR
the higher the serum creatinine, the worse the kidney function

Question 7

what affects serum creatinine?

Answer 7

muscle mass, diet, creatine supplements, malnutrition, and amputations.

Question 8

Cockroft – Gault formula- creatinine clearance

Answer 8

[(140-age) x lean body weight in kg] / (72 x serum creatinine)

Multiply x 0.85 for women

Question 9

MDRD formula use

Answer 9

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

Question 10

Describe the use of serum creatinin to measure kidney function in CKD

Answer 10

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

Question 11

What can ultrasound be used for in assessing renal disease?

Answer 11

Assess Kidney size
Morphology
Rule out obstruction

Question 12

What can CT be used for in assessing renal disease?

Answer 12

Provides complimentary information to that obtained
Ultrasound: for example to further characterize cysts

Diagnosis of kidney stones

Question 13

What occurs in acute kidney injury?

Answer 13

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.

Question 14

what is the clinical use of urine pH?

Answer 14

low urine pH may be associated with uric acid stones

calcium phosphate stones and UTIs with urea splitting organisms may result in high pH

Question 15

what can proteinuria tell us clinically?

Answer 15

abnormal basement membrane, leakage from tubules (Balkan nephropathy, tubulointerstitial nephritis) overflow (multiple myeloma - Bence Jones proteins)

Question 16

what an RBCs in urine tell us?

Answer 16

with proteinuria, possible intrinsic renal problem

Question 17

what can glucose in urine tell us?

Answer 17

renal disease

Question 18

what can ketones in urine tell us?

Answer 18

diabetic ketoacidosis, states of starvation

Question 19

What can urobilirubinogen tell us?

Answer 19

if obstructive jaundice –> urobilirubinogen decreased

if not obstructive –> urinary urobilirubinogen increased

Question 20

What can nitrite in urine tell us?

Answer 20

screen test for gram negative bacteria

ascorbate may produce false negative

Question 21

What can leukocytes in urine tell us?

Answer 21

glycosuria, high specific gravity, cephalexin, tetraycycline therapy

Question 22

what is a bland sedimentt?

Answer 22

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

Question 23

what is acute tubular necrosis sediment?

Answer 23

most common cause of acute renal failure, classically has tubular cells, granular debris, pigmented granular casts

Question 24

what is nephritic sediment?

Answer 24

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.

Question 25

what is nephrotic sediment?

Answer 25

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.

Question 26

what does urine in pyelonephritis and acute interstitial nephritis look like?

Answer 26

many WBCs and white blood cell casts. culture will be positive in pyelonephritis but not in acute interstitial nephritis.

Question 27

where do you see broad casts?

Answer 27

chronic renal failure in which tubules of functioning nephrons have dilated

Question 28

what are telescoped urine?

Answer 28

it has elements of chronicits (broad casts) and more acute disease (granular casts and RBC casts). Characteristic but not specific for rapidly progressive glomerulonephritis

Question 29

Crystals found in urine

Answer 29

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