2: Nonprotein Nitrogen Compounds

What are some compounds included in the NPN fraction of plasma and urine?

• urea

• uric acid

• creatinine

• ammonia

Which NPN compound is present in highest concentration in the blood?

urea

Which NPN compound:

• present in highest conc in the blood

• major excretory product of protein metabolism

• formed in the liver from amino groups and free ammonia generated during protein catabolism

urea

Measurement of which NPN is used to:

• evaluate renal function

• assess hydration status

• determine nitrogen balance

• assist in diagnosis of renal disease

• verify adequacy of dialysis

urea

elevated concentration of urea in blood (BUN)

azotemia

high plasma urea concentration and renal failure

uremia

What diseases are caused by:

• hemorrhage

• dehydration

• renal failure

• glomerular nephritis

• tubular necrosis

• renal calculi

• bladder tumors

azotemia & uremia

What is the normal BUN/creatinine ratio?

10:1 - 20:1

What is actually measured for urea?

NAD+

What specimens are used for urea measurement?

• plasma

• serum

• urine

What must be avoided when using plasma for urea testing?

• ammonium ions

• high concentrations of sodium citrate

• sodium fluoride

Why must ammonium ions, high concentrations of sodium citrate and sodium fluoride be avoided when using plasma for urea testing?

inhibit urease

product resulting from breakdown of purines in the liver

uric acid

When do uric acid crystals form?

in acidic urine (pH <5.5)

What specimens are used for uric acid measurement?

• heparinized plasma

• serum

• urine

What additives can’t be used for uric acid measurement?

EDTA or fluoride

What can cause falsely decreased uric acid?

hemolysis & high bilirubin

What is the coupled enzymatic method for measuring uric acid?

measure hydrogen peroxide produced when uric acid is converted to allantoin

MSU

monosodium urate

crystallized urate that’s relatively insoluble

MSU

What happens with high concentrations of MSU?

crystals may form and precipitate in the tissues and joints causing gout

Which NPN can cause gout?

uric acid

Which disease:

• severe pain in one or more joints

• joint feels hot and tender

• swelling in and around affected joint

• red, shiny skin over affected joint

gout

flow rate of filtered fluid through the kidneys

GFR

What is the most useful index of kidney fxn?

GFR

How is GFR measured?

estimated from creatinine in serum and urine

What test estimates the GFR?

creatinine clearance test

What creatinine/GFR results indicate renal damage?

elevated plasma creatinine & decreased GFR

waste product of creatine that results from using muscles

creatinine

chemical made to supply energy mainly to muscles

creatine

What NPN is related to muscle mass, the rate of creatine turnover, and renal fxn?

creatinine

most common method to measure creatinine

Jaffe method

Which method:

creatinine reacts with picric acid in alkaline soln to form a red-orange substance

Jaffe method

reference method for creatinine

IDMS

sources of error that falsely increase creatinine conc

• ascorbate

• glucose

• uric acid

• alpha-keto acids

• cephalosporin

Which NPN is produced during catabolism of AAs?

ammonia

Where is ammonia converted to urea?

liver

Which NPN:

• excreted by the kidneys and acts as a buffer for urine

• free __ is extremely toxic and is present in very low concs in plasma

ammonia

Which NPN is used to evaluate:

• Reye’s syndrome

• severe liver disease

• deficiencies of urea cycle enzymes

ammonia

acute metabolic disorder of the liver

Reye’s syndrome

When do you test for deficiencies of urea cycle enzymes in infants?

• unexplained nausea

• vomiting

• neurological deterioration assoc w/ feeding

What methods are used to measure ammonia?

GLDH & NADPH

Why must you analyze ammonia specimens ASAP?

ammonia conc increases rapidly following specimen collection

What anticoagulants are used for ammonia specimens?

heparin & EDTA

What are sources of error for ammonia measurement?

• cigarette smoking

• urine

• ammonia in detergents