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NPNs Flashcards

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1
Q

non-protein nitrogen compounds

A

compounds that contain nitrogen but are not proteins

ex. urea, creatinine, creatine, amino acids, uric acid, ammonia

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2
Q

Urea

A

***of all the NPNs, urea is present in highest concentration the blood

  • *Synthesized in the liver from amino groups (-NH2) & free ammonia from protein catabolism
  • catalyzed by enzymes in the urea cycle

protein–> amino acids–>ammonia–> urea

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3
Q

urea- biochemistry

A

urea is the major nitrogen-containing product of protein catabolism

more than 90% of urea is excreted through the kidneys
- minor losses through GI tract

Most urea in the glomerular filtrate is excreted in urine, but some is reabsorbed in the renal tubules

patients with kidney disease will have an increased level of urea in their blood ( Azotemia)

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4
Q

Urea vs BUN

A

historically, whole blood samples were used to measure nitrogen content & testing was called Blood Urea Nitrogen (BUN)

this is now an obsolete term, although it is still used in the United States
- Urea Nitrogen is a more appropriate term
( bc we use plasma not whole blood)

To convert between BUN & Urea :
BUNx 2.14 = Urea
Urea/2.14= BUN

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5
Q

Urea - specimen requirements

A

Can be measured on serum, plasma or urine

if plasma is collected:
- avoid ammonium ions
-do not use sodium citrate ( blue top) or sodium fluoride ( grey top) tubes
- citrate & fluoride inhibit urease ( enzyme we use
in measurement of urea)
avoid hemolysis

specimen are susceptible to bacterial decomposition ( particularly urine)
- refrigerate specimen if not analyzed within a few
hours ( 24hrs)

  • evaluate renal function w/urea
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6
Q

Urea- Analytical Methods

A

enzymatic ( urease)

Electrode

Isotope Dilution Mass Spectrometry (IDMS)
-Reference Method

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7
Q

Urea- Coupled Enzymatic Assay

A

urea is hydrolyzed with urease to produce ammonium ions (NH4+)
-ammonium ions ( what we quantify @ the end)

                  Urease Urea + 2H2O ------> 2NH4^+  + CO3^2-

                                 GLDH NH4^+ +2-oxoglutarate ------->gluatamate  + H2O
           NADH +  H+               NAD+

a decrease in absorbance of NAD+ is measured at 340nm
Can be measured as an endpoint or kinetic reaction

** more common on automated analyzers

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8
Q

Urea- Colorimetric method

A

**Urea in an acidic medium condenses with diacetly monoxime at 100 degrees to form a red colored complex

urea + diacetyl monoxime——> red colored complex

the intensity of color formed is directly proportional to the amount of urea present in the sample

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9
Q

Urea- Berthelot Method

A

Urease hydrolyses urea to ammonia & CO2
urease
Urea + H2O ———-> ammonia + CO2

ammonia reacts with a phenolic chromagen & hypochlorite to form a green colored complex

ammonia + Phenolic chromagen + hypochlorite (green colored complex )

the intensity of color produced is proportional to the amounnt of urea in the sample

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10
Q

Electrochemical Analysis of Urea

A

Urea is hydrolyzed by urease to ammoinum ions (NH4+) ***common step

2 methods to measure NH4+
1. Conductimetric:
measures the rate of change of conductivity ( inc. conductivity ) as the ion is formed

  1. Potentiometric :
    used in some point of care devices
    uses an ammonium ion-selective electrode with a membrane containing immobilized urease
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11
Q

Urea - reference ranges

A

plasma/serum : 2.1-7.1 mmol/L ( based on normal protein intake )

Urine : 12-20 g/day or 430-710 mmol/day

Urea levels gradually increase with age & are typically higher in men than women ( bc of muscle mass)

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12
Q

Clinical significance of urea

A

increase in urea in the blood is called Azotemia **

Very high levels with renal failure us called uremia or uremic syndrome ***( dialysis or transplant required)

Azotemia is classified into 3 main categories based on cause:
prerenal azotemia
renal azotemia
postrenal azotemia

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13
Q

Prerenal Azotemia

A 
caused by : 
Decreased renal blood flow 
- congestive heart failure 
- shock
-hemorrhage 
- dehydration 
( less blood is delivered to the kidney & therefore, less urea is filtered) **

Increased protein catabolism

  • stress, fever, major illness
  • corticosteroid therapy
  • GI hemorrhage
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14
Q

Renal Azotemia

A

caused by decreased renal function & compromised renal excretion of urea

  • acute or chronic renal failure
  • Glomerulornephritis
  • Tubular necrosis

very high plasma urea along with renal failure = uremia or uremic syndrome
- can be fatal if not treated by dialysis or kidney transplant

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15
Q

Postrenal Azotemia

A

caused by an obstruction of urine flow anywhere in the urinary tract

  • renal calculi
  • bladder or prostate tumors
  • severe infection
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16
Q

decreased urea

A

low protein diet
severe liver disease
late pregnancy or infancy
severe vomiting & diarrhea

17
Q

Urea nitrogen /creatinine ratio

A

ratio can help differentiate the cause of abnormal urea concentrations
***normal ratio 10:1 to 20:1

prerenal conditions :
elevated urea; normal creatinine - causes high ration

renal conditions :
both urea & creatinine elevated - normal ratio

postrenal conditions:
both elevated, urea elevated more - high ratio

low ratio is associated with low protein intake, acute tubular necrosis & severe liver disease

18
Q

urea as a diagnostic tool

A

blood urea can vary greatly with non- renal factors

not a good independent indicator of renal function

mostly used together with creatinine determination

19
Q

Creatine

A

**creatine is synthesized in the kidney, liver & pancreas from amino acids

**after synthesis, it is transported in the blood to other organs such as muscle & brain

**in muscle, creatine is phosphorylated to phosphocreatine, a high-energy compound

a small proportion of creatine spontaneously & irreversibly converts to creatinine ( its anhydride waste product)

20
Q

Creatinine

A

creatinine is a spontaneous decomposition product of creatine

the amount of creatinine produced is relatively constant & related to muscle mass

it is revived firm plasma via glomerular filtration & excreted in the urine

21
Q

Creatinine - specimen requirements

A

can be measured on serum, plasma or urine

hemolyzed & icteric samples should be avoided

high protein ingestion may temporarily elevate serum concentrationa

serum & urine creatinine is stable for 7 days at 4 degrees ( or freeze)

22
Q

creatinine - analytical methods

A

can be measured using:

chemical methods based on Jaffe reaction

enzymatic methods

  • **isotope dilution mass spectrometry
  • reference method
23
Q

Jaffe reaction for creatinine

A

creatinine reacts with picric acid in an alkaline solution to form a red-orange chromogen
NaOH
creatinine + picric acid——–> Creatinine picrate

main disadvantage: 
not specific for creatinine 
many non- creatinine Jaffe- like chromagens interfere 
- ascorbic acid 
-glucose 
-ketone bodies
- protein 
-cephalsporins ( antibiotics ) 
-pyruvate
24
Q

Approaches to increase specificity of Jaffe Reaction

A

increased accuracy can be obtained by using a protein- free filtrate absorbed onto Fuller’s earth ( aluminum magnesium silicate) or Lloyd’s reagent ( sodium aluminum silicate )

Increased specificity can be obtained by using:

  • a kinetic method ( Jaffe method) ***
  • an enzyme method ( Jaffe Method)
25
Q

Creatinine - kinetic methods

A

2 types of non creatinine chromogens hsve been identified in rate reactions

  • those that interfere within the first 20 seconds have been identified in rate reactions
  • those that interfere 80-100 seconds after mixing

the window between 20-80 seconds will be more specific for creatinine
- picrate reaction

used with automated instruments

26
Q

Creatinine - Enzymatic methods

A 
coupled enzymatic reactions involving : 
creatininase 
creatinase 
sarcosine oxidase
peroxidase

adapted to use on dry-slide analyzers ( like vitros)

27
Q

Sources of error

A 
at temps over 30 degrees the following can interfere: 
ascorbate 
glucose
⍺-ketoacids 
uric acid

bilirubin causes a negative bias

patients taking cephalosporin antibiotics may have falsely elevated results ( creatinine )

28
Q

Creatinine - Reference Ranges

A

serum/plasma : 50-110 µmol/L

urine 5-18 mmol/day

men will have higher creatinine levels than women (due to increased muscle mass)

29
Q

creatinine - clinical significance

A

elevated serum creatinine is associated with abnormal renal function
if serum creatinine is inc. glomerular filtration rate (GFR) is decreased.

increased with :
abnormal renal function

normal with:
muscle diseases

insensitive marker
- may not be noticably increased until kidney function is <50%

30
Q

Creatine- clinical significance

A 
increased in : 
muscle diseases 
- muscle dystrophy 
- poliomyelitis 
- hyperthyroidism 
-trauma

not increased in :
renal disease

we measure CK –> creatine kinase ( bc there is no good method to measure creatine directly )

31
Q

Kidney function test

A

serum/plasma urea

serum/plasma creatinine

creatinine clearance test

glomerular filtration rate

creatinine has the advantage over urea in that it is not affected by protein intake

creatinine clearance is the best test fro kidney/glomerular function

32
Q

Glomerular filtration rate (GFR)

A

a reliable measure of functional capacity of the kidneys

indicates the # of functioning nephrons

a decrease in GFR precedes all forms of progressive kidney disease

measuring GFR is useful to : 
target treatment 
monitor progression 
predict when renal replacement therapy (RRT) will be needed
- dialysis, transplants
33
Q

Glomerular filtration rate

A

GFR= [Us] x V
————
[Ps]

Us= urinary concentration pf the substance 
V= volumetric flow rate of urine in mL/min 
Ps= plasma concentration of the substance

labs are encouraged to report an estimated GFR (eGFR) when serum creatinine is ordered to increase identification of kidney disease

eGFR accounts for a patients body surface area, age, gender & ethnicity

34
Q

Creatinine clearance

A

renal clearance is “ the volume of plasma from which the substance is completely cleared by the kidneys per unit of time “

clearance depends on:

  • the net result of glomerular filtration & tubular reabsorption & secretion
  • is proportional to a BSA( body surface area)of 1.73 m^2

creatinine :

  • eliminated mainly by glomerular filtration
  • only a small amount is reabsorbed by the tubules
  • creatinine clearance is a good measure of glomerular filtration
35
Q

Creatinine clearance - specimen

A

precisely timed urine specimen
- 24 hr is most common

serum or plasma sample collected within 24 hrs of urine sample ***

creatinine is measured on both urine & serum

36
Q

creatinine clearance -calculation

A 
creatinine clearance = UV/P x 1.73/A
U = urine creatinine (µmol/L)
V= urine volume ( mL/s or mL/min)  
P= plasma creatinine (µmol/L)
A= body surface area ( m^2)
       - height & weight on requisition needed
37
Q

Creatinine clearance - sources of error

A

incorrect timing of urine collection

loss of urine during collection

vigorous excercise

patient must be properly hydrated

  • improves accuracy
  • eliminates retention of urine in the bladder as a source of energy
38
Q

Creatinine clearance - reference ranges

A

75-125 mL/min
1.24-2.08 mL/s

plasma creatinine level is inversely related to creatinine clearance
if patient has an incr. plasma creatinine, they will have a decr. creatinine clearance

chemistry (13 decks)

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