Chapter 66: Organ Function Tests

Question 1

Organ function tests (definition)

Answer 1

• Collective term for a variety of tests/procedures that evaluate organ function

Question 2

Primary uses of OFTs

Answer 2

• Detecting disease (less useful in detecting cause)
• Monitor response to treatment/assess prognosis
• Part of the diagnostics

Question 3

Shotgun approach

Answer 3

• Using numerous OFTs

- Confuses the investigation

Question 4

Organ disease be suspected when

Answer 4

• Results of at least two biochemically or physiologically independent tests generate abnormal results
• Compare results with reference range

Question 5

Idea OFT (though rarely available in routine practice)

Answer 5

• Correct result (accuracy) that is the same if repeated (precision)
• Measures low concentrations of the analyte (sensitivity)
• Not subject to interference by other substances (specificity)
• Should be cheap, simple and quick to perform readily automated
• Painless to patient, free of risk

Question 6

Specificity

Answer 6

• Probability that an OFT returns negative results and person is actually negative
• High specificity = less false positives

Question 7

Sensitivity

Answer 7

• Proportion of true positives detected by the organ function test
• High sensitivity = less false negatives

Question 8

Kidney functions

Answer 8

• Remove metabolic waste
• Water balance
• Maintain body fluid pH
• Excrete toxins/metabolites

Question 9

Rate of glomerular filtration

Answer 9

• 130 mL/min

- (125 mL/min for physio)

Question 10

Main types of kidney function tests

Answer 10

• Blood (plasma creatinine, BUN)
• Urinalysis
• Glomerular filtration rate (inulin, creatinine)

Question 11

Plasma creatinine evaluates

Answer 11

• Muscle turnover
• Abnormal men > 2.0 mg/dL
• Abnormal women > 1.5 mg/dL

Question 12

Healthy kidney filtration

Answer 12

• 30mg/L of protein (minimal in urine)

- 24 hour collection should contain no more than 150 mg of protein

Question 13

Proteinuria

Answer 13

• Presence of significant amounts of protein in the urine

- Indicates renal disease/glomerular damage

Question 14

Urinalysis (often first requested)

Answer 14

• Small sample or 24 hour collection
• Can reveal systemic issues too
• Complement w/ micro examination of sediment

Question 15

Urine collected for urinalysis should NOT contain

Answer 15

• Glucose
• Proteins
• Hemoglobin
• Metabolites

Question 16

Urinalysis ideal pH

Answer 16

• Acidic (5.0-6.0)

Question 17

Plasma creatinine

Answer 17

• Endogenous, derived from muscle tissue
• Preferred test of kidney function
• More specific than BUN

Question 18

Plasma creatinine production is dependent on

Answer 18

• Muscle mass

Question 19

Plasma creatinine removal from circulation

Answer 19

• Filtered by glomerulus

- Some secreted into filtrate by the proximal tubule (elevated if GFR falls)

Question 20

Plasma creatinine normal range

Answer 20

• Men: 0.8 – 1.2mg/dL

- Women: 0.6 – 0.9mg/dL (lower during pregnancy and in children)

Question 21

Elevated creatinine indicates impaired kidney function when

Answer 21

• It’s not filtered

- Decline in GFR

Question 22

Factors affecting accuracy of plasma creatinine

Answer 22

• Obesity
• Oedema
• Pregnancy
• Ascites

Question 23

Blood urea nitrogen (BUN) measures

Answer 23

• Amount of nitrogen in the urea
• Often performed alongside plasma creatinine
• May be used to predict pancreatitis

Question 24

Urea (mwt 60D)

Answer 24

• Relatively nontoxic waste product
• Produced in liver (breakdown of excess dietary protein/urea cycle)
• Filtered and excreted

Question 25

BUN reference range

Answer 25

- 7-20 mg/dL

Question 26

BUN/kidney function relationship

Answer 26

- As function decreases, BUN increases | - BUN elevated when renal function drops to 24-50% of normal

Question 27

A low BUN may be due to

Answer 27

- Lack of protein - Severe liver disease - Overhydration

Question 28

Creatinine clearance values (normal 24 hour collection)

Answer 28

- 90 - 139 ml/min men under 40 - 80 - 125 ml/min women under 40 - For people over 40, values decrease by 6.5 ml/min for each decade of life

Question 29

When GFR drops below 30 ml/min,

Answer 29

- Tubular secretion can exceed the amount filtered at the glomerulus

Question 30

Liver function tests

Answer 30

- Evaluate excretory or synthetic functions - Plasma assay of bilirubin - Prothrombin time/albumin

Question 31

Enzyme assays for liver function

Answer 31

- ALT - AST - AP - CGT

Question 32

AST leaks from injured

Answer 32

- Liver - Heart - Skeletal muscle - Erythrocytes (less often kidney)

Question 33

ALT is more specific for

Answer 33

- Liver (but less stable in blood samples)

Question 34

AST and ALT generally follow the same upward/downward courses in liver disease

Answer 34

- ALT rises less in alcoholism | - If ALT is greater than 300, suggests underlying problem is not alcoholism

Question 35

AST/ALT > 2

Answer 35

- Indicates alcoholism

Question 36

Bilirubin

Answer 36

- Breakdown product of hemoglobin - Normal levels usually below 1 mg/dL (majority unconjugated) - A variety of drugs may increase bilirubin

Question 37

Assay of bilirubin based on

Answer 37

- Van den Bergh reaction

Question 38

Jaundice is caused by

Answer 38

- Excess circulating bilirubin (> 2.5mg/dL)

Question 39

Elevated levels of unconjugated BR

Answer 39

- Liver cell problems | - Kernicterus

Question 40

Elevated levels of conjugated BR

Answer 40

- Obstruction either within the liver (intrahepatic) or outside the liver

Question 41

Ven den Bergh DIRECT reaction

Answer 41

- Direct bilirubin measurement | - Conjugated bilirubin can be expressed as direct bilirubin (readily assayed with azo dyes)

Question 42

Direct bilirubin reaction

Answer 42

- Water-soluble reagent (diazotized sulfanilic acid) reacts with conjugated bilirubin - Forms azobilirubin - Colorimetric assay at 600nm

Question 43

Normal direct bilirubin assay

Answer 43

- Trace amounts of conjugated (direct) bilirubin | - Elevations of water soluble, conjugated bilirubin leads to increased levels in the urine

Question 44

Van den Bergh INDIRECT reaction

Answer 44

- Additional accelerator agent added to free unconjugated bilirubin from serum albumin - Measures indirect or unconjugated bilirubin

Question 45

Normal indirect bilirubin assay

Answer 45

- Normal values vary with age | - Elevated unconjugated bilirubin (high affinity for membrane lipids) leads to impairment of cell membrane function

Question 46

Analysis of blood ammonia may be useful in diagnosis/management of

Answer 46

- Severe liver diseases - Hepatic encephalopathy - Liver failure - Heart failure - Hemolytic disease

Question 47

Measurement of blood ammonia levels are used to confirm a diagnosis of

Answer 47

- Reye's syndrome | - Together with the AST and ALT results

Question 48

Normal values for ammonia

Answer 48

- Vary widely | - Depends on laboratory, patient age, and type of specimen

Question 49

Tests of cardiac function

Answer 49

- Creatine kinase - Troponin T and I - C-reactive protein (CRP) - Myoglobin - B-type natriuretic peptide (BNP)

Question 50

C-reactive protein (CRP)

Answer 50

- A crude hepatic marker of chronic inflammation | - Elevated in atherosclerosis

Question 51

Possible causes of elevated CRP

Answer 51

- Genes connected with metabolic syndrome - Can also be used to predict MI or stroke - Often used in combination with CK-MB and cTnT

Question 52

Elevated myoglobin

Answer 52

- Detected in plasma and urine | - Indicative of cardiac function

Question 53

B-type natriuretic peptide (BNP) assay

Answer 53

- Used if congestive heart failure is suspected

Question 54

Routine urinalysis allows for determination of

Answer 54

- Physical, chemical and microscopic properties of urine