South College AVL Lab Med: Renal Function and Lipids - Lecture 6

Serum creatinine (SCr)

Waste product of muscle metabolism, proportional to muscle mass and excreted daily by kidneys.

BUN (blood urea nitrogen)

Product of digestion and protein metabolism (takes place in the liver), excreted by the kidneys

urea, creatinine

? - filtered and reabsorbed

? - filtered and secreted

eGFR (glomerular filtration rate)

simple test, calculated with serum creatinine, that estimates how well your kidneys are filtering waste from your blood - mainly used to check kidney function.

lower eGFR

a higher creatinine level usually means what for eGFR?

kidney function

BUN levels rise when what decreases?

NO! (but BUN levels often change in parallel w/ creatinine)

is eGFR directly calculated from BUN?

NEED TO KNOW HOW KIDNEY IS FUNCTIONING BEFORE PRESCRIBING MEDS!

Why is it important to obtain SCr, BUN, and eGFR?

AKI (acute kidney injury)

Abrupt decrease in renal function

retention of urea, dysregulation of extracellular volume and electrolytes, multiple etiologies can cause this

What is often seen with an AKI?

Increased SCr by >/= 0.3 (mg/dL) within 48 hours OR 50% increase SCr in 7 days

OR

Decrease urine output <0.5 (ml/kg/hr) for >6 hours

How can creatinine levels/urine output help us assess if someone has an Acute Kidney Injury (AKI)?

Acute Tubular Necrosis (ATN)

What is the most common cause of an AKI?

Ischemic ATN (most common)

Nephrotic ATN

What are the two types of ATN?

poor blood flow (shock, sepsis, severe dehydration) or kidneys don't get enough O2 (damaged tubules)

What causes with Ischemic ATN?

toxic substances - drugs (aminoglycosides, contrast dye, cisplatin), myoglobin (rhabdomyolosis), hemoglobin (hemolysis)

What causes with nephrotic ATN?

tubular cells die/slough off - block tubules

filtration impaired - waste builds up in the blood

What happens with an AKI? (INSERT PHOTO)

low urine output (oliguria) or sometimes normal

rising serum Cr and BUN

electrolyte imbalance (ex: hyperkalemia)

What are some symptoms of an AKI?

prerenal, intrarenal, postrenal

What are the three causes of acute renal failure?

prerenal

DECREASED BLOOD FLOW TO KIDNEYS

sudden and severe drop in pressure or interruption of blood flow to kidneys from severe injury or illness

Intrarenal

INTRINSIC KIDNEY DAMAGE

direct damage to the kidneys by inflammation, toxins, drugs, infection, or reduced blood supply

postrenal

OBSTRUCTION PREVENTING MICTURITION

sudden obstruction of urine flow due to enlarged prostate, kidney stones, bladder tumor, or injury

CHF, shock (sepsis, GI bleed), dehydration, vomitting/diarrhea w/ dehydration, ACE/ARBs and NSAIDs

What are some causes of prerenal acute renal kidney failure?

ATN, glomerulonephritis, pyelonephritis, interstitial nephritis, rhabdo, DM, nephrotoxic drugs (contrast, NSAIDS), Anabolic steroids

What are some causes of renal acute renal kidney failure?

prostate (BPH, cancer), nephrolithiasis (bilateral), bladder outlet obstruction, neurogenic bladder, tumor

What are some causes of post-renal acute renal kidney failure?

High Bun:Cr ratio (>20:1 - elevated)

Low Urine Na+ (<20 mmol/L - salt follows water)

What would we expect to see for prerenal Bun:Cr ratio and urine output?

Normal Bun:Cr ratio (10-15:1)

Low Urine Na+ (>20 mmol/L)

What would we expect to see for renal Bun:Cr ratio and urine output?

Variable Bun:Cr

What would we expect to see for post-renal Bun:Cr ratio and urine output?

More Cr produced from muscle metabolism

Why does an increase in muscle mass cause an increase on serum Cr?

muscle breakdown temporarily raises Cr

Why does an intense exercise cause a transient increase on serum Cr?

creatinine like substances absorbed from diet

Why does high protein or cooked meat intake cause a slight increase in serum Cr?

increased GFR (more blood filtered) clears Cr more rapidly

Why does pregnancy cause a decrease in serum Cr?

lower muscle mass = less Cr production

Why does aging cause a decrease/normal serum Cr?

AKI (sudden drop in GFR)

CKD (eGFR <60 mL/min for 3+ months)

Rhabdo (massive muscle breakdown - releases large SCr and myoglobin)

Urinary Tract obstruction (back pressure reduces filtration)

Heart failure or hypovolemia (reduce blood flow to kidneys - impair function)

Nephrotoxic drugs (direct kidney damage - reduces filtration)

Glomerulonephritis (inflammation of glomerulus reduces filtration capacity)

What are some disease processes that increase Serum Cr?

Muscle wasting (ex: malnutrition, muscular dystrophy - less Cr produced)

liver disease (liver makes Cr - if impaired, Cr production may drop)

overhydration (dilutes serum Cr)

What are some disease processes that decreases Serum Cr?

urinary symptoms (increase/decrease urine output, urgency, nocturia, incontinence, weak stream/straining, flank pain w/ gross hematuria, microscopic hematuria and/or proteinuria on urinalysis)

general symptoms (acute HTN or acute worsening HTN, peripheral/periorbital edema, symptoms of infection, arthralgia/myalgia, fever/chills, weight loss, night sweats, fatigue, etc.)

what are some acute reasons as to why we would measure creatinine levels?

CKD, DM, HTN, any autoimmune diseases affecting kidneys (lupus, sarcoidosis), any chronic condition requiring chronic meds

What are some chronic reasons as to why we should measure creatinine levels?

pre-renal AKI

(low Na, high K, normal Cl, low bicarb, high BUN, high BUN:Cr, high glucose, low Ca)

68 YO M patient w/ HTN and type 2 DM presents saying "I've been feeling weak and not peeing as much as usual".

BP: 90/60, HR: 105, dry mucus membranes, BMP: refer to photo

What are you suspecting this patient has?