Pancreas Function Tests - Biochemistry Exam Notes

First, understand the pancreas quickly

Pancreas function tests mostly focus on the exocrine part, because that is what biochemistry labs can measure.

Big Picture: Two Categories of Tests

PANCREAS FUNCTION TESTS
        |
   _____|_____
   |         |
INVASIVE   NONINVASIVE
(need a    (tubeless -
  tube)     easier but
           less accurate)

CATEGORY 1: INVASIVE TESTS (Direct Tests)

A. Secretin Test

• What you do: Give secretin (a hormone) by IV injection
• What you measure: Volume of juice + bicarbonate (HCO3-) output in duodenum
• What it tells you: Tests the ductal cells of the pancreas (which make bicarbonate and water)
• Normal: Bicarbonate should rise significantly after secretin
• In disease: Bicarbonate output drops = pancreatitis, pancreatic cancer

B. CCK (Cholecystokinin) Test

• What you do: Give CCK (or synthetic ceruletide) by IV
• What you measure: Enzymes in duodenum - amylase, lipase, trypsin, chymotrypsin
• What it tells you: Tests the acinar cells (which make enzymes)
• In disease: Enzyme output is reduced

C. Secretin + CCK Test (GOLD STANDARD)

• Best of both worlds: Combines both tests above
• Measures volume + bicarbonate + all enzymes together
• Most sensitive and specific test for exocrine pancreatic function
• Drawback: Needs duodenal intubation + IV hormones, not available everywhere

D. Lundh Test Meal (historical)

• Patient eats a standardized meal (6% fat, 15% carbohydrate, 74% non-nutrient fiber)
• Meal stimulates pancreas naturally (no IV hormone needed)
• Trypsin concentration measured in duodenal fluid
• No longer used routinely - mainly of historical interest

CATEGORY 2: NONINVASIVE TESTS (Indirect / Tubeless Tests)

Key fact to remember: The pancreas has a huge reserve. Malabsorption only appears when enzyme output falls to 5-10% of normal. So tubeless tests miss mild disease.

A. Fecal Elastase-1 (Most important noninvasive test!)

• What: Measure elastase-1 enzyme in a stool sample
• Why it's good: Elastase-1 is stable as it passes through the gut (not broken down)
• Result:
  • Normal: >200 mcg/g stool
  • Severe insufficiency: <100 mcg/g stool
• Uses: Screening for pancreatic insufficiency, especially in cystic fibrosis children and chronic pancreatitis
• This is the most commonly used noninvasive test today

B. Fecal Chymotrypsin

• Measure chymotrypsin in stool
• Less reliable than elastase (chymotrypsin can be degraded in the gut)
• Only detects severe dysfunction

C. Fecal Fat (72-hour stool collection)

• Patient eats 100g fat/day for 3 days, all stool collected
• Normal: <7g fat per day in stool
• Steatorrhea (disease): >7g fat/day
• Confirms malabsorption but does not pinpoint pancreas as the cause
• Inconvenient for patient (stool collection for 3 days!)

D. NBT-PABA Test (Bentiromide Test)

• Patient swallows N-benzoyl-L-tyrosyl-para-aminobenzoic acid (NBT-PABA) with a meal
• Chymotrypsin in the gut cleaves it → releases PABA
• PABA is absorbed, enters blood, excreted in urine
• Measure PABA in blood or urine = indirect measure of chymotrypsin activity
• Problem: Can be falsely abnormal if the small intestine is damaged (not specific to pancreas)

E. Fluorescein Dilaurate Test (Pancreolauryl Test)

• Similar idea to PABA test
• Patient eats fluorescein dilaurate + a test meal
• Pancreatic esterase cleaves it → fluorescein absorbed → measured in urine/serum
• Same problem: small bowel disease can give false positives

Quick Comparison Table (memorize this!)

SERUM ENZYME TESTS (Used for Acute Pancreatitis Diagnosis)

Serum Amylase

• Rises within 2-12 hours of acute pancreatitis
• Peaks at 12-72 hours
• Returns to normal in 3-5 days
• Problem: Not specific (can rise in salivary gland disease, intestinal obstruction, renal failure)

Serum Lipase

• Rises within 4-8 hours
• Stays elevated longer than amylase (8-14 days)
• More specific to the pancreas than amylase
• Preferred test for acute pancreatitis today

Rule of thumb: Amylase = quick rise, quick fall. Lipase = slower rise, stays longer, more specific.

Clinical Conditions Where These Tests Are Used

Quick Memory Tricks

1. "DIRECT tests = BEST results" - direct tests need a tube but give the most accurate answer
2. "Secretin = Bicarbonate, CCK = Enzymes" - secretin stimulates juice/bicarb, CCK stimulates enzyme release
3. "Elastase stays alive in stool" - that is why fecal elastase is the best tubeless test
4. "Lipase outlasts amylase" - lipase stays high longer in acute pancreatitis
5. "Pancreas has huge reserve" - tests only become abnormal when 90-95% of function is lost (for tubeless tests)

One-Line Summary for Each Test

• Secretin test = give secretin IV, measure bicarbonate in duodenum
• CCK test = give CCK IV, measure enzymes in duodenum
• Secretin + CCK = gold standard, measures everything
• Fecal elastase = best noninvasive test, stool sample, stable enzyme
• Fecal fat = steatorrhea confirmation, 72-hr stool, not pancreas-specific
• NBT-PABA = oral substrate, urine PABA measures chymotrypsin
• Serum amylase = rises fast in acute pancreatitis, not specific
• Serum lipase = rises slower, more specific, preferred marker

• Pancreas at a glance (exocrine vs endocrine side by side)
• Two main categories explained (invasive vs noninvasive)
• Full invasive tests table (Secretin, CCK, Gold Standard combo, Lundh)
• Memory trick for Secretin vs CCK

• Full noninvasive tests table (Fecal Elastase, Fecal Fat, NBT-PABA, Fluorescein Dilaurate)
• Serum enzyme tests for acute pancreatitis (Amylase vs Lipase)
• Master comparison table - all tests in one place
• Which test for which disease (clinical applications)
• 8 memory tricks box
• Key numbers to memorize (normal values, timing, thresholds)

Creatinine Estimation & Urea vs Serum Creatinine - Biochemistry Exam Notes

First: Where Do These Come From?

Creatinine - Where It Comes From

Creatine (stored in muscle)
        |
        | (broken down during muscle metabolism)
        |
   Creatinine
        |
        | (filtered by kidneys)
        |
   Excreted in urine

• Creatinine is a waste product of muscle metabolism
• It is produced at a constant, steady rate every day
• It depends on your muscle mass - more muscle = more creatinine
• It is freely filtered by the glomerulus (the kidney's filter)
• Very little is reabsorbed - mostly just filtered and thrown out

Urea (BUN = Blood Urea Nitrogen) - Where It Comes From

Dietary Protein / Body Protein
        |
        | (broken down in liver)
        |
   Amino Acids
        |
        | (deamination - liver)
        |
   Ammonia (toxic!) --> Liver converts it to UREA (safe)
        |
        | (urea goes to blood)
        |
   Filtered by kidneys --> Excreted in urine

• Urea is the end product of protein breakdown
• Made in the liver from ammonia
• Amount depends hugely on how much protein you eat

THE BIG QUESTION: Why is Serum Creatinine PREFERRED Over Urea?

Short answer: Serum creatinine is a MORE RELIABLE marker of kidney function than blood urea (BUN).

Creatinine - Advantages (Why It Is Preferred)

• Adult men: 0.6 - 1.2 mg/dL
• Adult women: 0.5 - 1.1 mg/dL
• Upper limit (older textbooks): 1.4 mg/100 mL

Urea (BUN) - Disadvantages (Why It Is NOT Preferred Alone)

Factors that RAISE BUN (without kidney disease):

1. High protein diet - eat a lot of meat, BUN goes up
2. Fever / infection - body breaks down more protein (catabolism)
3. Glucocorticoids (steroids) - increase protein breakdown
4. Tetracycline antibiotics - antianabolic effect
5. GI bleeding (upper) - blood in gut is digested as protein, raises BUN
6. Dehydration - low blood volume causes more urea reabsorption
7. Old age - more catabolism

Factors that LOWER BUN (without kidney improvement):

1. Low protein diet / starvation / malnutrition - less protein = less urea
2. Liver disease - liver cannot convert ammonia to urea properly
3. Pregnancy - dilutional effect

Because BUN is affected by so many outside factors, it does NOT reliably reflect kidney function alone. Serum creatinine is preferred.

How Creatinine Is Measured - The Methods

Method 1: Jaffe Method (Alkaline Picrate Method) - Most Common Historically

Creatinine + Alkaline Picrate (sodium picrate in NaOH)
                    |
              Orange-red colour
                    |
        Measured by colorimetry (spectrophotometry)

1. Patient's serum is added to alkaline picrate solution
2. Creatinine reacts with picric acid in alkaline conditions
3. Forms an orange-red complex (Janovsky complex)
4. Colour intensity is measured by colorimeter/spectrophotometer
5. Compare to known standard = calculate creatinine concentration

• Overestimates creatinine by ~25% because other substances also react with picrate (these are called "non-creatinine chromogens")
  • Glucose, proteins, acetoacetate (ketones), bilirubin, ascorbic acid, pyruvate
• High bilirubin causes falsely LOW readings
• Diabetic patients are especially affected (high glucose + ketones interfere)
• Modified Jaffe methods try to remove these interfering substances first

Method 2: Enzymatic Method - More Accurate (Now Recommended)

Creatinine
    |  (creatininase enzyme)
    v
Creatine
    |  (creatine amidino-hydrolase)
    v
Sarcosine + Urea
    |  (sarcosine oxidase)
    v
Glycine + Formaldehyde + H2O2
    |  (peroxidase + dye)
    v
Coloured product -> Measured photometrically

• More specific for creatinine - fewer interferences
• Less affected by glucose, proteins, bilirubin
• Expert bodies and KDIGO recommend this method

Method 3: IDMS (Isotope Dilution Mass Spectrometry) - Reference Method (Gold Standard)

• The most accurate method - reference standard
• All lab methods should be calibrated to IDMS
• Not used routinely in hospitals (too complex/expensive)
• Used to standardize other methods

Method 4: HPLC (High-Performance Liquid Chromatography)

• Very specific and accurate
• More analytical specificity than conventional methods
• Severe limitation: low throughput (too slow for routine use)

Creatinine Clearance - The Functional Test

Creatinine Clearance (mL/min) = (Urine Creatinine x Urine Volume) / Serum Creatinine

CrCl = (U_cr x V) / P_cr

• U_cr = urine creatinine concentration (mg/dL)
• V = urine volume in mL/min (usually from 24-hour collection)
• P_cr = plasma/serum creatinine (mg/dL)

Important exam fact: Kidneys have a large functional reserve - 70% of kidney function must be lost before renal failure shows up in tests!

eGFR - Estimated GFR (Calculated from Serum Creatinine)

• Uses serum creatinine + age + sex + race
• Standard for reporting eGFR in adults

CrCl = [(140 - age) x weight in kg] / (72 x serum creatinine)
       Multiply by 0.85 for females

BUN:Creatinine Ratio - Exam Favourite!

Limitations of Serum Creatinine (Do NOT Ignore These!)

1. Muscle mass effect - bodybuilders have high creatinine naturally; elderly/malnourished people have low creatinine even with bad kidneys
2. Late marker - GFR must fall to ~50% before serum creatinine clearly rises (due to compensatory hyperfiltration)
3. Tubular secretion - some creatinine is secreted by tubules (not just filtered), which slightly overestimates GFR
4. Drug interference - trimethoprim and cimetidine block tubular secretion of creatinine, raising serum creatinine WITHOUT actual kidney damage
5. Jaffe method interferences - as described above

Acute Kidney Injury (AKI) - Creatinine Criteria

1. Serum creatinine rises by ≥0.3 mg/dL within 48 hours
2. Serum creatinine rises to ≥1.5 times baseline within 7 days
3. Urine output <0.5 mL/kg/hour for 6 hours

Quick Comparison: Urea vs Serum Creatinine

Memory Tricks

1. "Creatinine is CONSTANT, Urea is UNSTABLE" - creatinine comes from muscle at a steady rate; urea depends on what you ate
2. "BUN = BIG Uncertainty Nitrogen" - too many things affect it
3. "Jaffe = Orange fake" - Jaffe method gives falsely high reading (fake creatinine from glucose/proteins)
4. "IDMS = I Don't Miss Specificity" - IDMS is the gold standard, most accurate
5. "10:20 ratio is normal - above 20 = pre-RENAL (dehydrated)" - BUN:Cr ratio >20 = prerenal problem
6. "70% kidney loss before tests go abnormal" - kidneys have huge reserve
7. "Creatinine rises 1-2 mg/dL per day in anuric AKI"
8. "Trimethoprim tricks you" - raises creatinine without damaging the kidney

One-Line Summary for Each Test

• Serum creatinine = preferred kidney marker, constant production, reflects GFR
• Jaffe method = alkaline picrate + colorimetry, overestimates by 25%, interferences from glucose/proteins/bilirubin
• Enzymatic method = more specific, fewer interferences, now recommended
• IDMS = gold standard reference method, not routine
• Creatinine clearance = actual GFR measurement from 24-hr urine + serum creatinine, normal 72-140 mL/min
• eGFR (CKD-EPI) = estimated GFR from serum creatinine formula, no urine needed
• BUN = not preferred alone, affected by diet/liver/dehydration/GI bleed
• BUN:Cr ratio = very useful to find cause: >20 = prerenal, normal = intrinsic renal, <10 = low protein/liver

• Where creatinine vs urea come from (side-by-side origin boxes)
• Why serum creatinine is preferred - full comparison table (8 factors)
• All factors that raise/lower BUN falsely (colour-coded red/green)
• Jaffe method - principle, steps, interferences, what causes false highs/lows
• Enzymatic method - full enzyme cascade reaction, why it's better
• IDMS and HPLC methods summary table

• Methods comparison table (all 4 methods side by side)
• Creatinine clearance formula + eGFR (CKD-EPI + Cockcroft-Gault)
• BUN:Creatinine ratio table (>20 = prerenal, 10-20 = intrinsic renal, <10 = low protein/liver)
• 7 limitations of serum creatinine
• 14 key numbers to memorize (normal values, cutoffs, AKI criteria)
• 11 memory tricks box