Renal Function Flashcards
1
Q
Blood urea nitrogen
- relationship to GFR
- define uremia and azotemia
A
- urea is filtered and partially reabsorbed by the nephron, which means that BUN always slightly underestimates the GFR
- Reabsorption increases with hypovolemia; thus BUN underestimates the GFR even more in hypovolemic states
- An increase in BUN = azotemia
- uremia refers to the toxic effects of elevated BUN
2
Q
GFR calculation
A
- Creatinine passes freely through glomerulus and a small amount is secreted by tubules; the latter increases with increasing serum [Cr]
- Creatinine overestimates GFR
ClearanceCr = UCr/PCr x VUr/time(minutes)
UCr is urine creatinine (mg/dL)
VUr is volume of urine (mL, collected over 24 hours)
PCr is plasma creatinine (mg/dL)
Units for ClCr are mL/min with 80-120 mL/min being normal
3
Q
Problems with using creatinine clearance to estimate GFR
A
- relationship between GFR and creatinine is nonlinear: mild impairment in GFR does not cause much increase in creatinine
- When GFR is 1/2 normal, creatinine begins to linearly reflect changes in GFR
- Nonglomerular influences upon creatinine; creatinine concentration is increased by muscle mass, muscle activity, muscle injury (trauma, surgery), and protein intake
- Creatinine decreases with age and is influenced by race, sex, medications
4
Q
Other calcuations for GFR
A
-
Modification of Diet in Renal Disease (MDRD) Study equations is most widely used, but is being replaced by:
- Chronic Kidney Disease Epidemiologic Collaborative (CKD EPI) equation in many labs
- MDRD equation is invalid at high GFR (eGFR over 60 mL/min/1.73 m2)
- CKD EPI equation is valid over the whole eGFR range
5
Q
BUN/Creatinine ratio
A
- Normal is 10:1
- Prerenal azotemia: ratio is increased, frequently > 20:1, in renal hypoperfusion (2/2 hypovolemia, hypotension)
- Ratio is maintained near normal in intrarenal causes of renal failure
6
Q
Cystatin C
A
- Freely filtered by glomerulus and completely reabsorbed by proximal tubule
- at least as good as serum creatinine for estimating the GFR and less dependent on age, sex, or muscle mass
- Strong predictor of cardiovascular mortality in patients with chronic renal disease
7
Q
Normal proteinuria
A
Does not exceed 150 mg/day (mainly Tamm-Horsfall protein)
8
Q
Significant proteinuria
- level
- method
- compared to creatinine clearance
- dipstick
A
- > 300 mg/day
- based on 24 hour urine collection
- random urine samples (“spot urine”)
- can be misleading because protein handling by kidney varies throughout the day
- when compared to simultaneous urine creatinine determination, the spot urine protein measurement is as good as the 24 hour urine
- urine dipstick result (1+ to 3+) is semiquantitative and most sensitive to albumin; not sensitive enough to detect microalbuminuria
9
Q
Microalbumin assay
A
- capable of detecting as little as 0.3 mg/dL of albumin (dipstick sensitive to ~30 mg/dL)
- microalbuminuria defined by albumin:creatinine ratio (mg/G) of a spot urine rather than a 24 hour collection
- possible source of confusion - protein:creatinine ratio is reported as mg/mg
10
Q
urine Beta2 microglobulin and lysozyme assays
A
- these proteins are freely filtered by the glomerulus and then completely reabsorbed by the normally functioning proximal convoluted tubule
- presence in the urine suggests tubular dysfunction
11
Q
Laboratory screening for chronic kidney disease
- who gets screened
- what tests are done
- define CKD
A
- Annual testing for those at high risk for CKD, including patients with:
- diabetes mellitus
- HTN
- family history of renal disease
- High risk groups should get
- eGFR
- microalbuminuria screen (urine albumin: creatinine ratio)
- Chronic kidney disease defined by:
- GFR <60 mL/minute/1.73 m2 of body surface area
- albuminuria for 3 or more consecutive months
- Stage categorizations:
- Stage 1: kidney damage (albuminuria) without decreased GFR (GFR > 90 mL/min/1.73 m2 or dialysis)
- Stage 2: kidney damage with a mild decrease in GFR (GFR 60-89 mL/min/1.73 m2 or dialysis)
- Stage 3: moderate decrease in GFR (GFR 30-59 mL/min/1.73 m2 or dialysis)
- Stage 4: severe decrease in GFR (GFR 15- 29 mL/min/1.73 m2 or dialysis)
- Stage 5: renal failure (GFR < 15 mL/min/1.73 m2 or dialysis)
12
Q
Laboratory evaluation in acute renal failure
Prerenal ARF
A
- Result of decreased renal perfusion
- A sustained benefit by expansion of intravascular volume with colloid is characteristic
- BUN/Cr ratio usually elevated
- Fractional excretion of sodium (FENa) low (<1%)
- “Inactive” urine sediment
13
Q
Laboratory evaluation in acute renal failure
postrenal ARF
A
- bilateral obstruction of the renal collecting system
- BUN:Cr ratio often elevated
14
Q
Laboratory evaluation in acute renal failure
Intrarenal ARF
A
- Injury to the nephron (glomeruli, tubules, vessels, or interstitium)
- ATN is most common cause of intrarenal ARF
- Most common causes of ATN are
- ischemia
- nephrotoxins
- acute glomerulonephritis
- Usually normal BUN/Cr ratio
- FENa > 1%
- Urine may show “active” sediment:
- dysmorphic RBCs and RBC casts in glomerulonephritis
- coarse granular casts in ATN, GN, or interstitial nephritis
- WBC casts in pyelonephritis
- eosinophils in acute allergic interstitial nephritis
15
Q
Acute renal failure (ARF): prerenal vs renal
A
18
Q
Causes of postrenal acute renal failure
A
