Clinical Chemistry and Renal Assessment (25-26)

What are the functions of the kidney?

Filtration

Secretion (proximal tubule)

Reabsorption

What happens in the filtration process of the kidneys?

~180 L/day; proteins >60 kDa and protein-bound drugs are not filtered

What is secreted in the kidneys?

Creatinine

What happens during the reabsorption process of the kidneys?

glucose fully reabsorbed until serum >/ 180 mg/dL → glucosuria

What is GFR?

Key marker used to stage Chronic Kidney Disease

Why do we use GFR?

Quantifies kidney function (filtration capacity)

Most kidney functions decline in parallel with GFR

A falling GFR is an early and reliable indicator of CKD progression

What is the Diagnosis role of GFR in CKD?

persistent reduction in GFR

<60 for more than 3 months = CKD

What is the staging role of GFR in CKD?

determines CKD stage

What is the prognosis role of GFR in CKD?

lower GFR → higher risk of complications (CV disease, ESRD, mortality)

What is the therapy guidance role of GFR in CKD?

influences drug dosing, timing of nephrology referral and dialysis/transplant planning

What is the filtrate that GFR can be directly measured with?

24-CrCl

What is Serum Creatinine?

Filtered, not reabsorbed;

~ 10 - 20% secreted in proximal tubule (increase percentage as GFR falls → CrCl overestimates true GFR)

What are non-GFR factors that influence SCr (GFR unchanged)?

Medications

Extreme weight

Muscle mass

pregnancy

rapidly changing kidney function

protein intake

vitamins

diurnal variations

What is Cystatin C?

produced by all nucleated cells;

filtered, not secreted; reabsorbed and metabolized in proximal tubule

What are non-GFR factors that affect Cystatin C levels?

thyroid dysfunction

steroids

inflammation

smoking

class III obesity

When do you use the eGFRcyc?

When SCr is significantly affected by non-GFR factors making it unreliable

When do you use 24-hour Creatinine clearance?

Comsider in pregnancy, rapidly changing kidney function or when non-GFR determinants affect SRc/CysC make the estimations equations invalid

What nephrotoxic drugs do we wait to start when we are checking baseline renal function?

Vancomycin

Chemotherapy

Contrast dye

When is CKD-EPI more accurate than MDRD-4?

When eGFR >60

What is CKD-EPI?

The standard for diagnosing and monitoring of renal dysfunction

What is the normal BUN level?

8-20mg/dL

When BUN >20mg/dL, what does it suggest?

Pre-renal cause of decreasing GFR

What is the prerenal BUN:SCr ratio?

>20:1

What is the normal BUN/SCr ratio?

10-15:1

What is the intrinsic BUN:Scr ratio?

<10:1

What is the collection/handling process of a Urinalysis?

clean-catch, midstream; analyze promptly

What happens when a urinalysis collection is delayed?

decreased glucose

ketones evaporate

bacteria multiply

What is indicated when the urinalysis smells fruity?

Diabetes Ketoacidosis

What does it mean when the Urinalysis smells turbid?

WBC

RBC

crystals

What does a WBC casts mean?

Intrarenal inflammation/infection

What does a RBC cast mean?

glomerulonephritis/ischemia

What do Hylan casts mean?

present during periods of renal blood flow and suggest pre-renal condition

What do tubular epithelial cells mean?

intrarenal cause of kidney dysfunction

What do muddy granular cells mean?

Acute Tubular Necrosis

What does a Leukocyte esterase mean from a Urinalysis?

if positive, UTI/inflammation

Vit C can cause false negative

What is a Nitrite from a Urinalysis?

produced by Gram-negative bacteria

Positive nitrite → Gram (-) UTI

When is diabetes indicated in a Urinalysis?

when serum >180 mg/dL and ketones are seen

When is Prerenal indicated in Fractional Excretion of Sodium?

<1%

When is Intrinsic indicated in FeNa?

>2%

What is Anuria?

little to no pee

<100mL/day

What is Oliguria?

low urine output

<400mL/day

often prerenal

What is the Urine Sediment that indicates prerenal dysfunction?

Hyaline casts, may be normal

What is the Urine Sediment that indicates Intrinsic dysfunction?

Muddy brown granular casts; tubular epithelial casts

What is the Urine Sediment that indicates Postrenal dysfunction?

variable, may be normal