18 - Renal Function Test and Urinary Analysis Flashcards Preview

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Flashcards in 18 - Renal Function Test and Urinary Analysis Deck (53)
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What are the tests you can do to assess renal function?

- Lab tests (BUN, creatinine, electrolytes - mainly K, but also Na and bicarb)
- Urinalysis (dip-stick, urine microscopy, electrolytes)
- Imaging studies (ultrasound, CT, functional studies)


Describe creatinine

- Byproduct of muscle creatine
- Nearly 100% FILTERED, minimally re-absorbed


What is the clinical utility of creatinine?


- Because of it’s filtration, reabsorption pattern, it is a useful marker of GFR
- Baseline values dependent on MUSCLE MASS
- Not a perfect test – inaccurate in mild impairment, no utility in CKD, individual variability******

EMPHASIZED that it is NOT a perfect test


Describe BUN

Marker of nitrogen/urea content of the blood


What does a rise in BUN mean?


Rise in BUN may indicate:
- Kidney injury/reduction in GFR
- Increased nitrogen load (UGI bleed, high-protein diet)
- Dehydration (less plasma = ↑ concentration)
- Steroid use

EMPHASIZED that if you eat a lot of meat and eggs, you can have high BUN

Need to exclude the other possibilities before you say there are kidney problems


What does low BUN mean?

- liver failure
- urea cycle defects


What is the clinical utility of BUN levels?


*Clinical Utility – used in conjunction with creatinine to assess renal function

**** Do NOT just look at one thing... Look at both of these ****


Describe what the BUN to Cr ratio tells you


Classically, this has been used to distinguish the origins of an acute kidney injury
- BUN:Cr > 20 suggests PRE-RENAL
- BUN:Cr


Describe testing electrolytes for kidney function

The kidney is responsible for maintaining electrolyte balance via filtration, reabsorption, and secretion


Describe potassium levels

Potassium – worry about increase, but low also a concern


Describe phosphorous levels

Phosphorus – elevation a concern in CKD


Describe sodium levels

Sodium – changes are related to disorders of WATER, not Na


Describe other electrolyte levels

- Bicarbonate – provides a clue in certain renal or endocrine causes for AKI
- Magnesium – check in hypokalemia
- Chloride – useful in confusing acid-base cases (calculating Anion Gap)
- Calcium – CKD, certain cases of AKI


Describe GFR estimation methods

Most common/easiest – serum creatinine
- Accuracy affected by mass, baseline, other factors
- Requires correction equations (MDR, Cockcroft-Gault)

More accurate options
- Inulin excretion – requires injection, monitoring
- 24h urine creatinine – difficult execution
- These are not very convenient


Describe a urinalysis


Clues on dip-stick
- Specific gravity – hydration status, renal concentrating capacity
- Proteinuria – mild, moderate, severe
* Test detects higher MW proteins (albumin)
- Infectious changes – leukocyte esterase, nitrites
- Blood – hematuria, rhabdomyolysis
* Test detects blood PROTEINS, not RBC’s
- Sugar, ketones - diabetes

***Normal UA = no blood, no nitrites, no ketones, no glucose, no leukocyte esterase***


What is on your list of differentials for hematuria

- IgA nephropathy
- Post-infectious nephropathy
- Hereditary nephropathy (Alports syndrome)
- Thin basement membrane disease
- Malignancy
- Nephrolithiasis
- Infection
- Foley trauma


Describe urine microscopy in the clinical setting

- Should ALWAYS be done when dip-stick is abnormal
- Normal microscopy – no “casts”, no RBC, no WBC, few or no epithelial cells


Describe abnormal findings on a urine microscopy


- WBC – infection, sterile pyuria
- Epithelial cells – may indicate sample contamination
- RBC – correlate with dip-stick blood
- Casts


Describe the abnormal finding of casts


Casts – cells coated with tubular proteins
- ***“Muddy brown” granular - Acute Tubular Necrosis
- Hyaline casts - Dehydration/volume depletion
- ***RBC – glomerulonephritis, vasculitis, renal infarct
- ***WBC – pyelonephritis, interstitial nephritis, post-strep GN
- Fatty, “maltese cross” – nephrotic syndrome
- Pigment – hemolysis, rhabdomyolysis, liver disease

Anytime you see "casts" it means that it has come from the tubules

This is important to make a good diagnosis, but no one can remember all of it, will need to reference later (?)


Describe the interpretation of a UA or micro

- Dip stick may indicate infection, heavy proteinuria or volume depletion
- Microscopy may help distinguish between different RENAL causes


What are the "pearls" of interpreting UA or micro?


- Muddy brown cast = ATN
- RBC cast = GN
- WBC cast = pyelo or AIN
- Eosinophils = AIN, contrast
- Dipstick blood pos, no RBC = Rhabdomyolysis
- ***No protein on dip, proteinuria on UPC ratio = Multiple myeloma (SSA can be added to turn dipstick pos)


What are the imaging studies we can use to look at the kidneys?

- CT
- Ultrasound


Describe the use of ultrasound

Cheap, easy, non-invasive, safe

You can see and diagnose...
- Bladder distension/urinary retention
- Hydronephrosis – distal obstruction
- Perinephric fluid collections – abscess, hematoma
- Kidney size –> atrophy = CKD
- Cysts – PCKD, tumors


Describe the use of CT

More expensive, radiation exposure, possible contrast exposure

Reasons to perform CT over ultrasound:
- To diagnose nephro- or urolithiasis
- Body habitus prevents quality ultrasound images
- Further delineation of lesions seen on ultrasound
- Angiography necessary


Describe AKI

(not emphasized)

Acute Kidney Injury
- Acute, rapid loss of renal function


What is the diagnostic criteria for AKI?

(not emphasized)

- Creatinine rise >=0.3mg/dL in =50% OR...
- UOP (urine osmotic pressure) =6hrs


Describe the severity of AKI

(not emphasized)

Occurs along a spectrum from mild injury to fulminant failure requiring dialysis (RRT=Renal Replacement Therapy)


How do you classify AKI?


3 categories based on the anatomic or functional location of the problem
- Pre-renal
- Post-renal
- Intra-renal


Describe case 1

42yo previously healthy male presents to the ED with confusion, lethargy, and fever over the past 3 days. He has also had nausea and vomiting for the past 24h. His family reports he has not had anything to eat or drink for the past 48h and is very weak.

PMH: HTN, hyperlipidemia

Medications: Lisinopril, Chlorthalidone,Simvastatin

No other significant social, surgical, or family history is reported.


Describe the physical exam for case 1

- VS – 99.4, 122, 18, 105/60, 98% on RA
- Abdomen moderately tender, no peritoneal signs, hyperactive bowel sounds