Chapter 66: Organ Function Tests Flashcards
Organ function tests (definition)
- Collective term for a variety of tests/procedures that evaluate organ function
Primary uses of OFTs
- Detecting disease (less useful in detecting cause)
- Monitor response to treatment/assess prognosis
- Part of the diagnostics
Shotgun approach
- Using numerous OFTs
- Confuses the investigation
Organ disease be suspected when
- Results of at least two biochemically or physiologically independent tests generate abnormal results
- Compare results with reference range
Idea OFT (though rarely available in routine practice)
- 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
Specificity
- Probability that an OFT returns negative results and person is actually negative
- High specificity = less false positives
Sensitivity
- Proportion of true positives detected by the organ function test
- High sensitivity = less false negatives
Kidney functions
- Remove metabolic waste
- Water balance
- Maintain body fluid pH
- Excrete toxins/metabolites
Rate of glomerular filtration
- 130 mL/min
- (125 mL/min for physio)
Main types of kidney function tests
- Blood (plasma creatinine, BUN)
- Urinalysis
- Glomerular filtration rate (inulin, creatinine)
Plasma creatinine evaluates
- Muscle turnover
- Abnormal men > 2.0 mg/dL
- Abnormal women > 1.5 mg/dL
Healthy kidney filtration
- 30mg/L of protein (minimal in urine)
- 24 hour collection should contain no more than 150 mg of protein
Proteinuria
- Presence of significant amounts of protein in the urine
- Indicates renal disease/glomerular damage
Urinalysis (often first requested)
- Small sample or 24 hour collection
- Can reveal systemic issues too
- Complement w/ micro examination of sediment
Urine collected for urinalysis should NOT contain
- Glucose
- Proteins
- Hemoglobin
- Metabolites
Urinalysis ideal pH
- Acidic (5.0-6.0)
Plasma creatinine
- Endogenous, derived from muscle tissue
- Preferred test of kidney function
- More specific than BUN
Plasma creatinine production is dependent on
- Muscle mass
Plasma creatinine removal from circulation
- Filtered by glomerulus
- Some secreted into filtrate by the proximal tubule (elevated if GFR falls)
Plasma creatinine normal range
- Men: 0.8 – 1.2mg/dL
- Women: 0.6 – 0.9mg/dL (lower during pregnancy and in children)
Elevated creatinine indicates impaired kidney function when
- It’s not filtered
- Decline in GFR
Factors affecting accuracy of plasma creatinine
- Obesity
- Oedema
- Pregnancy
- Ascites
Blood urea nitrogen (BUN) measures
- Amount of nitrogen in the urea
- Often performed alongside plasma creatinine
- May be used to predict pancreatitis
Urea (mwt 60D)
- Relatively nontoxic waste product
- Produced in liver (breakdown of excess dietary protein/urea cycle)
- Filtered and excreted
BUN reference range
- 7-20 mg/dL
BUN/kidney function relationship
- As function decreases, BUN increases
- BUN elevated when renal function drops to 24-50% of normal
A low BUN may be due to
- Lack of protein
- Severe liver disease
- Overhydration
Creatinine clearance values (normal 24 hour collection)
- 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
When GFR drops below 30 ml/min,
- Tubular secretion can exceed the amount filtered at the glomerulus
Liver function tests
- Evaluate excretory or synthetic functions
- Plasma assay of bilirubin
- Prothrombin time/albumin
Enzyme assays for liver function
- ALT
- AST
- AP
- CGT
AST leaks from injured
- Liver
- Heart
- Skeletal muscle
- Erythrocytes (less often kidney)
ALT is more specific for
- Liver (but less stable in blood samples)
AST and ALT generally follow the same upward/downward courses in liver disease
- ALT rises less in alcoholism
- If ALT is greater than 300, suggests underlying problem is not alcoholism
AST/ALT > 2
- Indicates alcoholism
Bilirubin
- Breakdown product of hemoglobin
- Normal levels usually below 1 mg/dL (majority unconjugated)
- A variety of drugs may increase bilirubin
Assay of bilirubin based on
- Van den Bergh reaction
Jaundice is caused by
- Excess circulating bilirubin (> 2.5mg/dL)
Elevated levels of unconjugated BR
- Liver cell problems
- Kernicterus
Elevated levels of conjugated BR
- Obstruction either within the liver (intrahepatic) or outside the liver
Ven den Bergh DIRECT reaction
- Direct bilirubin measurement
- Conjugated bilirubin can be expressed as direct bilirubin (readily assayed with azo dyes)
Direct bilirubin reaction
- Water-soluble reagent (diazotized sulfanilic acid) reacts with conjugated bilirubin
- Forms azobilirubin
- Colorimetric assay at 600nm
Normal direct bilirubin assay
- Trace amounts of conjugated (direct) bilirubin
- Elevations of water soluble, conjugated bilirubin leads to increased levels in the urine
Van den Bergh INDIRECT reaction
- Additional accelerator agent added to free unconjugated bilirubin from serum albumin
- Measures indirect or unconjugated bilirubin
Normal indirect bilirubin assay
- Normal values vary with age
- Elevated unconjugated bilirubin (high affinity for membrane lipids) leads to impairment of cell membrane function
Analysis of blood ammonia may be useful in diagnosis/management of
- Severe liver diseases
- Hepatic encephalopathy
- Liver failure
- Heart failure
- Hemolytic disease
Measurement of blood ammonia levels are used to confirm a diagnosis of
- Reye’s syndrome
- Together with the AST and ALT results
Normal values for ammonia
- Vary widely
- Depends on laboratory, patient age, and type of specimen
Tests of cardiac function
- Creatine kinase
- Troponin T and I
- C-reactive protein (CRP)
- Myoglobin
- B-type natriuretic peptide (BNP)
C-reactive protein (CRP)
- A crude hepatic marker of chronic inflammation
- Elevated in atherosclerosis
Possible causes of elevated CRP
- Genes connected with metabolic syndrome
- Can also be used to predict MI or stroke
- Often used in combination with CK-MB and cTnT
Elevated myoglobin
- Detected in plasma and urine
- Indicative of cardiac function
B-type natriuretic peptide (BNP) assay
- Used if congestive heart failure is suspected
Routine urinalysis allows for determination of
- Physical, chemical and microscopic properties of urine
