Part 2: RNTs and Non-Protein Nitrogen Substances

What methods are used to measure Uric Acid?

• Uricase Method (enzymatic method)

  • UV or colorimetric detection

What is the principle of the Uricase Method for measuring uric acid?

• uric acid is oxidized by uricase

uric acid+ O2 + H2O → allantoin + CO2 + H2O2

Describe UV detection method for the Uricase Method for measuring uric acid?

• direct measurement of decrease in uric acid absorbance at 293 nm

Describe the Colorimetric Method for the Uricase Method for measuring uric acid?

• hydrogen peroxide reacts with chromogen → colored compound

• measured spectrophotometrically

What are the advantages of the Uricase Method?

• highly specific

• widely used in clinical laboratories

What are the limitations/interferences of the Uricase Method?

• ascorbic acid (reduces H2O2 → false low results)

• bilirubin (spectral interference)

What are the methods used to measure ammonia?

• GLDH (Glutamate Dehydrogenase) Method

What is the principle of the GLDH method?

• ammonia reacts with a-ketoglutarate in the presence of GLDH

NH3+ a-ketoglutarate + NADPH → glutamate + NADP

What absorbance measurement is used for the GLDH mathod?

• decrease in NADPH absorbance at 340 nm

  • rate of decrease is proportional to ammonia concentration

What are the advantages of the GLDH method to measure ammonia?

• highly specific

• suitable for automated analyzers

What are pre-analytical considerations that need to be taken into account for ammonia testing?

• use HIGHLY SENSITIVE HANDELING

  • use heparinized plasma

  • avoid hemolysis

  • transport sample on ice immediately

  • analyze promptly (within 30 min)

What is the normal adult BUN?

• 7 - 20 mg/dL

What is the normal adult creatinine?

• 0.6 - 1.3 mg/dL

What is the normal adult uric acid?

• 3.5 - 7.2 mg/dL

What is the normal adult ammonia?

• 15-45 ug/dL

What renal causes can increase Blood Urea Nitrogen (BUN) Azotemia?

• acute kidney injury (AKI)

• chronic kidney disease (CKD)

What pre-renal causes can increase Blood Urea Nitrogen (BUN) Azotemia?

• dehydration

• shock

• heart failure (reduced renal perfusion)

What post-renal causes can increase Blood Urea Nitrogen (BUN) Azotemia?

• urinary obstruction (stones, tumors)

What are other causes of increased Blood Urea Nitrogen (BUN) Azotemia?

• high protein diet

• gastrointestinal bleeding

• increased catabolism (trauma or fever)

What causes a decreased BUN?

• liver cirrhosis

• severe malnutrition

• over hydration

What is important to keep in mind when using a BUN test?

• it is sensitive but not specific

  • must be used alongside creatinine

what can cause increased serum creatinine?

• acute kidney injury (AKI)

• chronic kidney disease (DKI)

• urinary obstruction

What can very small increases of creatinine indicate?

• significant reduction in GFR

Which is less effected by diet, BUN or serum creatinine?

• serum creatinine

What can caused decreased creatinine?

• low muscle mass (elderly)

• pregancy (increased GFR)

What is a more specific and reliable indication of renal function than BUN?

• serum creatinine

What is a normal BUN: creatinine ratio?

• 10:1 or 20:1

a BUN: creatinine ratio of > 20: 1 indicates what condition?

• pre-renal azotemia

  • increased renal reabsorption ( cause could be dehydration)

a BUN creatine ratio of 10-20:1 indicates what?

• normal or renal azotemia

  • intrinsic kidney damage

A varied BUN: creatinine ratio indicates what?

• post renal azotemia

  • obstruction of urine flow

What can cause increased uric acid (hyperuricemia)?

• gout

• renal insufficiency (reduced excretion)

• increased cell turnover (leukemia, chemotherapy)

What can cause decreased uric acid?

• rare, but can occur in certain metabolic or genetic conditions

What can cause increased ammonia?

• hepatic encephalopathy

• severe liver dysfunction

• urea cycle disorders

What do elevated levels of ammonia indicate?

• failure of ammonia detoxification

What is it important to consider clinically when evaluating ammonia levels?

• symptoms correlate poorly with level - interpret alongside clinical findings

What are the different pattern-based interpretation that are useful clinically?

• Pre-renal

• Intrinsic Renal

• Post-renal

• Liver disease

What are causes of the pre-renal pattern values?

• reduced renal perfusion (dehydration) is most common

• heart failure, GI bleeds, diuretic use, shock, high protein intake

What values indicate a pre-renal pattern?

• ↑ BUN

• normal or ↑ creatinine

• ↑ ratio (> 20: 1)

What values indicate an Intrinsic renal pattern?

• ↑ BUN

• ↑ creatinine

• normal ratio (10-20:1)

What indicates a Post-renal Pattern?

• ↑ BUN

• ↑ creatinine

• variable ratio

What can cause a post-renal pattern?

• urinary obstruction

what results indicate a liver disease pattern and are seen in cirrhosis?

• ↓ BUN

• ↑ ammonia

Define Creatinine Clearance (CrCl).

• volume of plasma that is completely cleared of creatinine per unit time

What serves as a practical estimate of glomerular filtration rate (GFR), which is a strong indicator of kidney function.

• Creatine clearance (CrCl)

What is the formula for Creatine Clearance?

Ccr = (Ucr x V) / (Pcr x t)

• Ucr = urine creatine conc. (mg/dL)

• Pcr = plasma creatine conc. (mg/dL)

• V = total urine volume (ml)

• t = time of urine collection in minutes ( typically 1440 min for 24 hrs)

What is the equation for total creatinine excreted?

• Ucr x V

What is the equation for plasma concentration over time?

• Pcr x t

What is the Creatine clearance calculation used for?

• calculating how much plasma is “cleared” of creatinine per minute

What are the different specimen requirements that can be used for measuring creatine clearance?

• urine

• blood

What is the gold standard in routine labs?

• 24 hour urine collection

  • timed collections can be used as an alternative in a hospital collection

What is a blood sample used for?

• measuring serum creatinine collected

  • during or at midpoint of urine collection period

What is the reference range for creatine clearance in an adult male?

• 90 - 140 ml/min

What is the reference range for reatine clearance in an adult female?

• 80-125 ml/min

What is the clinical significance of using a creatine clearance test?

• more sensitive than serum creatinine alone

• detects early decline in GFR even if creatinine is still normal

A CrCl value of > 90 ml/min indicates what?

• normal kidney function

A CrCl value of 60-89 ml/min indicates what?

• mild decrease

A CrCl value of 30-59 ml/min indicates what?

• moderate impairment

A CrCl value of 15-29 ml/min indicates what?

• severe impairment

A CrCl value of <15 ml/min indicates what?

• kidney failure

What are limitations associated with creatinine clearance?

• overestimation of GFR

• collection errors

• biological variability

What can affect creatinine clearance values?

• muscle mass

  • ↑ in muscular people

  • ↓ in elderly

• diet

  • ↑ in high meat diet

• physical activity

What is used in most laboratories due to the limitations of urine collection?

• estimated GFR (eGFR)

What variables are used when calculating eGFR?

• serum creatinine

• age

• sex

• race

What are common eGFR equations?

• CKD-EPI (most common)

• MDRD (old equation)

What specimen can be used to measure creatine clearance?

• blood and urine

What specimen can be used to measure eGFR?

• Blood ONLY

Compare the accuracy of creatine clearance and eGFR.

Creatinine clearance

• more direct

eGFR

• estimate

Compare the Convienience of creatine clearance and eGFR.

Creatinine clearance

• low

eGFR

• high

Compare the clinical use of creatine clearance and eGFR.

Creatinine clearance

• special cases

eGFR

• routine screening