Liver function test (Add mnemonic if possible)

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liver function tests LFT mnemonic complete guide

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Liver Function Tests (LFTs)

Mnemonic: "ABCDEFGHI" - a popular way to remember all LFT components:

Letter Test
A Albumin / A:G ratio / Ammonia / Alpha-1 antitrypsin
B Bilirubin (Total / Direct / Indirect) / Bile pigments / Bile salts
C Cholesterol / Coagulation (PT, INR)
D De Ritis ratio (AST:ALT)
E Enzymes (ALT, AST, ALP, GGT)
F Fibrinogen / Flocculation tests
G GGT (Gamma-glutamyltransferase)
H Hepatitis markers (HBsAg, anti-HCV)
I INR / Immunoglobulins

Letter	Test
A	Albumin / A:G ratio / Ammonia / Alpha-1 antitrypsin
B	Bilirubin (Total / Direct / Indirect) / Bile pigments / Bile salts
C	Cholesterol / Coagulation (PT, INR)
D	De Ritis ratio (AST:ALT)
E	Enzymes (ALT, AST, ALP, GGT)
F	Fibrinogen / Flocculation tests
G	GGT (Gamma-glutamyltransferase)
H	Hepatitis markers (HBsAg, anti-HCV)
I	INR / Immunoglobulins

Overview

The term "liver function tests" is actually a misnomer - many of the tests do not assess liver function per se, but instead identify the source and pattern of liver damage. A standard LFT panel typically includes: bilirubin, ALT, AST, ALP, GGT, albumin, total protein, and PT/INR.

Tests Grouped by Category

1. Tests of Liver Injury (Enzymes)

ALT (Alanine Aminotransferase)

More specific for liver disease than AST
AST is elevated in cardiac/skeletal muscle injury; ALT is not
Markedly elevated in acute viral hepatitis (elevated days before jaundice appears)
Primary use: sensitive indicator of hepatocellular damage

AST (Aspartate Aminotransferase)

Less specific (also elevated in heart, skeletal muscle)
AST > ALT suggests:
- Alcoholic liver disease (AST:ALT ratio typically >2:1 - the De Ritis ratio)
- Early hepatic injury
- Cirrhosis (mild elevations)
- Non-hepatic muscle injury
Both AST and ALT >10x upper reference limit (URL) = acute hepatitis; lower values = chronic hepatitis

ALP (Alkaline Phosphatase)

Elevated in obstructive (cholestatic) jaundice
Also elevated in bone disease (not liver-specific alone - confirm with GGT)
If ALP elevated in isolation, rule out non-hepatic sources (bone, placenta, intestine)
ALP >2x URL with AST <3x URL = cholestatic pattern

GGT (Gamma-Glutamyltransferase)

Rises in parallel with ALP in cholestasis
Used to confirm that elevated ALP is of hepatic origin (not bone)
Elevated by alcohol and enzyme-inducing drugs
Prognostic indicator for cardiovascular and all-cause mortality; rising GGT associated with type 2 diabetes and metabolic syndrome

2. Tests of Hepatic Synthetic Function

Albumin

Low in chronic liver disease (cirrhosis, chronic hepatitis)
Half-life ~20 days, so does not reflect acute changes quickly
Decreased albumin = indicator of chronicity and severity
Can also be low in malnutrition, nephrotic syndrome, inflammation (not liver-specific)
Serial measurements monitor disease progression

Prothrombin Time (PT) / INR

Liver synthesizes clotting factors I, II, V, VII, IX, X
Prolonged PT in acute liver failure (impaired synthesis)
Key differentiator: if PT corrects after vitamin K injection (10 mg SC/IM, measure after 24h) - suggests cholestasis (malabsorption of fat-soluble vitamin K), not hepatocellular failure
Indicator of severity of liver disease

3. Tests of Bilirubin Metabolism

Serum Bilirubin (Total, Direct, Indirect)

Conjugated (direct) bilirubin elevated in: obstructive jaundice, cholestasis, hepatocellular disease
Unconjugated (indirect) bilirubin elevated in: hemolysis, Gilbert syndrome, Crigler-Najjar
In hepatocellular disease: both conjugated and unconjugated rise (failure of uptake, conjugation, and excretion)
In obstructive jaundice: mainly conjugated rises

Quick rule: Conjugated = Can't get out (obstruction) Unconjugated = Can't get in/conjugated (hemolysis, enzyme deficiency)

4. Other Tests

Test	Significance
Total Protein	Low in cirrhosis, chronic liver disease
Ammonia	Elevated in hepatic encephalopathy (impaired urea cycle)
Serum Cholesterol	Elevated in cholestasis; low in severe hepatocellular failure
Alpha-1 Antitrypsin	Deficiency causes cirrhosis and emphysema
LDH	Non-specific; elevated in hepatitis and many other conditions

Interpretation Patterns

Pattern	Key Finding	Likely Cause
Hepatocellular	AST/ALT >> ALP	Viral hepatitis, alcoholic hepatitis, drug-induced
Cholestatic	ALP/GGT >> AST/ALT	Bile duct obstruction, PBC, PSC
Mixed	Both elevated	Drug-induced, early/resolving hepatitis
Synthetic failure	Low albumin, prolonged PT	Cirrhosis, acute liver failure

Diagnostic Algorithm

Algorithm for using abnormal LFTs to classify liver disease

Reading the algorithm:

AST >3x URL + ALP <2x URL → Hepatocellular disease
- Normal albumin → Acute hepatitis
- Low albumin → Chronic hepatitis
AST <3x URL + ALP >2x URL → Cholestatic disease
- Then use ultrasound/cholangiography to differentiate intrahepatic vs. extrahepatic cholestasis

Quick Memory Aid: LFT Pattern Mnemonics

Hepatocellular injury - "A Liver Attack":

ALT and AST rise markedly (>10x = acute hepatitis)

Cholestasis - "ALP and GGT Go Up":

Think "Blocked Bile" - ALP and GGT rise when bile can't flow

De Ritis Ratio (AST:ALT):

>2:1 = Alcoholic liver disease
<1:1 = Viral hepatitis or NAFLD

Synthetic function - "PACT":

PT prolonged
Albumin low
Coagulation factors impaired
Think chronic disease or severe acute failure

Reference Ranges (approximate)

Test	Normal Range
ALT	7-56 U/L
AST	10-40 U/L
ALP	44-147 U/L
GGT	9-48 U/L
Total Bilirubin	0.2-1.2 mg/dL
Direct Bilirubin	0.0-0.3 mg/dL
Albumin	3.5-5.0 g/dL
Total Protein	6.3-8.2 g/dL
PT	11-13.5 seconds

Sources: Tietz Textbook of Laboratory Medicine, 7th Ed.; Harper's Illustrated Biochemistry, 32nd Ed.; Sleisenger and Fordtran's Gastrointestinal and Liver Disease

How do I interpret abnormal LFT patterns in specific diseases (hepatitis, cirrhosis, cholestasis)?

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Interpreting Abnormal LFT Patterns in Specific Diseases

The Two Core Patterns - Master This First

The most important first step in any abnormal LFT is determining whether the pattern is hepatocellular or cholestatic. This single distinction directs the entire workup.

Feature	Hepatocellular	Cholestatic
Dominant rise	ALT, AST (>ALP)	ALP, GGT (>ALT, AST)
Key threshold	AST >3× URL, ALP <2× URL	AST <3× URL, ALP >2× URL
Bilirubin	Both conjugated + unconjugated raised	Mainly conjugated raised
Albumin/PT	Severely impaired in acute failure	May be normal initially
Think of	Hepatitis, drug toxicity, ischemia	Obstruction, PBC, PSC, drugs

Disease-by-Disease LFT Interpretation

1. Acute Viral Hepatitis (A, B, C, D, E)

Hallmark: Dramatic ALT/AST elevation - typically >500 U/L, often >10-25× upper limit of normal (ULN)

Test	Finding
ALT	Markedly elevated (≥ AST); rises days before jaundice
AST	Elevated, but ALT ≥ AST
ALP	Mildly elevated or normal
Bilirubin	Elevated (both fractions) once jaundice appears
Albumin	Normal (acute = no time to deplete)
PT/INR	Normal unless severe/fulminant

Key rule: ALT ≥ AST in viral hepatitis. Values >25× ULN are almost exclusively seen in acute hepatocellular disease.

Caution on Hep C: ALT fluctuates in chronic HCV and can be normal even when AST is elevated - do not miss it.

2. Alcoholic Hepatitis

Hallmark: Moderate enzyme elevation with a distinctive AST:ALT ratio

Test	Finding
AST	Elevated, but rarely >300 U/L
ALT	Lower than AST
AST:ALT ratio	≥2:1 (De Ritis ratio reversal) - in severe cases up to 4:1
GGT	Often markedly elevated (elevated in 70% of chronic alcoholics; correlates with alcohol intake)
Bilirubin	Elevated
PT	May be prolonged

Mnemonic: "Two Shots to One" - AST is TWO times ALT in alcoholic hepatitis.

Why AST>ALT in alcohol? Alcohol depletes pyridoxal phosphate (vitamin B6) - ALT requires more B6 to be active than AST does. Also, alcohol directly damages mitochondria, releasing mitochondrial AST.

Distinguishing from viral hepatitis:

Alcoholic: AST:ALT ≥2:1, AST rarely >300 U/L
Viral: ALT ≥ AST, transaminases often >500-1000 U/L

3. Non-Alcoholic Fatty Liver Disease (NAFLD) / NASH

Test	Finding
ALT	Mildly-moderately elevated (usually <5× ULN)
AST	May be normal or mildly elevated; ALT>AST initially
ALP/GGT	Mildly elevated
Albumin/PT	Normal unless cirrhosis has developed

Key point: NAFLD is one of the most common causes of incidentally elevated liver enzymes in the Western world. Do NOT diagnose NAFLD on enzymes alone - suspect it when metabolic risk factors (obesity, T2DM, hypertension, dyslipidemia) are present.

When AST > ALT in NAFLD: Suggests progression to cirrhosis (ratio inverts as fibrosis develops).

4. Drug-Induced Liver Injury (DILI)

Two patterns exist:

Type	LFT Pattern	Examples
Hepatocellular	ALT/AST >5× ULN dominant	Paracetamol, isoniazid, statins
Cholestatic	ALP >2× ULN dominant	Chlorpromazine, erythromycin estolate, anabolic steroids, OCP
Mixed	Both elevated	Many drugs

Predictable (dose-dependent): Classic example = paracetamol overdose. Causes extreme transaminase rise (>2000 U/L). Extreme elevation of LDH alongside extreme AST/ALT signals massive hepatic necrosis.
Idiosyncratic: Not dose-dependent; minority of patients. Examples: isoniazid, halothane.

Hy's Law: If both ALT >3× ULN AND bilirubin >2× ULN in DILI - high risk of acute liver failure (mortality ~10%).

5. Cirrhosis (End-Stage Liver Disease)

Counterintuitive finding: Transaminases may be normal or only mildly elevated in cirrhosis because few functional hepatocytes remain to release enzymes.

Test	Finding	Significance
ALT/AST	Normal or mildly elevated	Few hepatocytes left to lyse
AST:ALT ratio	>1 (reversal)	Fibrosis causes this flip
ALP/GGT	Elevated (biliary involvement)
Albumin	Low	Best marker of chronicity/severity
PT/INR	Prolonged	Synthetic failure
Bilirubin	Elevated	Reflects severity
Platelets	Low (hypersplenism)	Portal hypertension
Ammonia	Elevated	Hepatic encephalopathy

Mnemonic for cirrhosis: "LAPP" - Low albumin, AST>ALT, Prolonged PT, Platelets low

Child-Pugh Score uses LFTs to grade severity:

Bilirubin, Albumin, PT, plus Ascites and Encephalopathy
Score A (5-6) = compensated; Score C (10-15) = decompensated

6. Obstructive (Extrahepatic) Cholestasis

(e.g., gallstones, pancreatic cancer, bile duct stricture)

Test	Finding
ALP	Markedly elevated (often >3-4× ULN)
GGT	Elevated in parallel with ALP
Conjugated bilirubin	Markedly elevated ("Can't get out")
ALT/AST	Mildly elevated or normal
Albumin	Normal early
PT	May be prolonged - corrects with vitamin K (malabsorption of fat-soluble vitamins)

PT correction with Vitamin K = cholestasis, not hepatocellular failure. This is the key clinical differentiator.

Next step: Ultrasound - dilated bile ducts confirm extrahepatic obstruction; then CT/MRCP.

7. Intrahepatic Cholestasis

(e.g., Primary Biliary Cholangitis - PBC, Primary Sclerosing Cholangitis - PSC, drug-induced)

Test	Finding
ALP	Markedly elevated
GGT	Elevated
Bilirubin	Elevated (conjugated)
ALT/AST	Mildly elevated
Anti-mitochondrial Ab (AMA)	Positive in PBC (>90% sensitivity)
IgM	Elevated in PBC
ANCA, ANA	May be positive in PSC/AIH

PBC: Middle-aged woman, itching, fatigue, elevated ALP + AMA positive.
PSC: Young man with IBD, elevated ALP + beaded appearance on MRCP.

8. Ischemic Hepatitis ("Shock Liver")

Hallmark: Explosive transaminase rise

Test	Finding
AST/ALT	Extreme elevation >2000-10,000 U/L (can mimic paracetamol overdose)
LDH	Also extremely elevated
Bilirubin	Rises later
PT	Prolonged

Occurs with acute hypoperfusion (cardiogenic shock, septic shock, massive PE). Transaminases rise within 1-3 days then fall rapidly when perfusion is restored.

LDH + ALP both elevated = think malignant hepatic infiltration (metastases, lymphoma). LDH rises disproportionately.

9. Wilson Disease

Test	Finding
AST:ALT	Elevated, often AST > ALT
ALP	Paradoxically LOW or normal (unique!)
Bilirubin	Elevated (Coombs-negative haemolytic anemia)
Ceruloplasmin	Low

Low ALP in a young patient with hepatitis = think Wilson disease. This is a classic exam clue.

Summary Table: LFT Patterns at a Glance

Disease	ALT/AST	ALP/GGT	Bili	Albumin	PT	Key Clue
Acute viral hepatitis	↑↑↑ (ALT≥AST)	Normal/↑	↑	Normal	Normal	ALT>500 U/L
Alcoholic hepatitis	↑↑ (AST≥2×ALT)	↑ GGT	↑	↓	↑	AST:ALT ≥2:1, AST<300
NAFLD/NASH	↑ (ALT>AST)	↑ mild	Normal/↑	Normal	Normal	Metabolic risk factors
Cirrhosis	Normal/↑ mild (AST>ALT)	↑	↑	↓↓	↑↑	Low albumin + reversed ratio
Extrahepatic obstruction	Normal/↑ mild	↑↑↑	↑ conj	Normal	↑ (corrects with vit K)	Dilated ducts on US
PBC/PSC	↑ mild	↑↑↑	↑ conj	↓ late	↑ late	AMA+ (PBC), IBD (PSC)
Ischemic hepatitis	↑↑↑↑	↑	↑	↓	↑↑	LDH also extreme; rapid fall
Wilson disease	↑↑ (AST>ALT)	↓ or normal	↑	↓	↑	Young patient, low ceruloplasmin
Liver metastases	↑ mild	↑↑↑	↑	↓	↑	LDH + ALP both ↑↑

Quick Interpretation Algorithm

Abnormal LFTs
     |
     ├── ALT/AST >> ALP?  → HEPATOCELLULAR
     │        |
     │        ├── ALT ≥ AST + >500 U/L  → Viral / toxic hepatitis
     │        ├── AST:ALT ≥2:1, AST <300 → Alcoholic hepatitis
     │        ├── AST:ALT >1, low albumin → Cirrhosis
     │        ├── Extreme >2000 U/L      → Ischemic or paracetamol
     │        └── AST>ALT + LOW ALP      → Wilson disease
     │
     └── ALP/GGT >> ALT/AST? → CHOLESTATIC
              |
              ├── PT corrects with Vit K  → Extrahepatic obstruction
              │        └── US → dilated ducts → stone/stricture/tumour
              └── Normal ducts on imaging → Intrahepatic cholestasis
                       ├── AMA+           → PBC
                       ├── IBD history    → PSC
                       └── Drug history   → Drug-induced cholestasis

Sources: Harrison's Principles of Internal Medicine, 22nd Ed.; Miller's Anesthesia, 10th Ed.; Quick Compendium of Clinical Pathology, 5th Ed.; Tietz Textbook of Laboratory Medicine, 7th Ed.

Generate a diagnostic flowchart poster (ALP vs AST for rapid diagnosis)

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A clean medical diagnostic flowchart poster titled "LFT Rapid Diagnosis: ALP vs AST Pattern" for clinical use. The poster uses a white background with a professional medical color scheme (deep navy, crimson red, teal green, amber yellow). At the top: bold title "LFT RAPID DIAGNOSIS" with subtitle "ALP vs AST Pattern Recognition" The flowchart starts with a central box "ABNORMAL LIVER FUNCTION TESTS" in navy blue. It splits into TWO main branches with arrows: LEFT BRANCH (crimson/red): - Box: "AST > 3× URL AND ALP < 2× URL" - Label below: "HEPATOCELLULAR PATTERN" - Splits into two sub-branches: LEFT: "Normal Albumin" → green box "ACUTE HEPATITIS" with bullet points: "• Viral (ALT ≥ AST, >500 U/L)", "• Toxic/Drug-induced", "• Ischemic (>2000 U/L)" RIGHT: "Low Albumin" → orange box "CHRONIC HEPATITIS / CIRRHOSIS" with bullets: "• AST:ALT ratio >1", "• Alcoholic (AST:ALT ≥2:1)", "• NASH/NAFLD" RIGHT BRANCH (teal/green): - Box: "AST < 3× URL AND ALP > 2× URL" - Label below: "CHOLESTATIC PATTERN" - Splits into two sub-branches: LEFT: "PT corrects with Vit K" → teal box "EXTRAHEPATIC CHOLESTASIS" with bullets: "• Gallstones", "• Pancreatic cancer", "• Bile duct stricture" and note "→ Ultrasound: dilated ducts" RIGHT: "Normal ducts on imaging" → purple box "INTRAHEPATIC CHOLESTASIS" with bullets: "• PBC (AMA+)", "• PSC (IBD history)", "• Drug-induced" At the bottom: a SPECIAL CASES section in amber/yellow banner: "⚠ SPECIAL PATTERNS: Wilson Disease = LOW ALP + elevated AST in young patient | Liver Mets = LDH + ALP both markedly elevated | Shock Liver = extreme AST/ALT >2000 U/L, rapid rise and fall" Bottom footer: "De Ritis Ratio (AST:ALT): >2:1 = Alcoholic | <1 = Viral/NAFLD | >1 with low albumin = Cirrhosis" The flowchart has clean arrows, rounded rectangle boxes, clear hierarchy, bold labels, and is suitable for printing as an A3 clinical reference poster.

LFT Rapid Diagnosis: ALP vs AST Diagnostic Flowchart Poster

Here's your diagnostic flowchart poster. It covers:

Two core branches - Hepatocellular (AST >3× URL, ALP <2× URL) vs Cholestatic (ALP >2× URL, AST <3× URL)
Sub-classification using albumin level (acute vs chronic hepatocellular) and PT response to Vitamin K (extra- vs intrahepatic cholestasis)
Special patterns highlighted at the bottom - Wilson disease (paradoxically low ALP), liver metastases (LDH + ALP both elevated), and shock liver (extreme transaminases >2000 U/L)
De Ritis ratio guide as a quick footer reference

You can right-click and save the image for printing as an A3 clinical reference. Let me know if you'd like any section expanded, recolored, or reformatted!

Kidney function test (add mnemonic if possible) Identification of abnormalities

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Kidney Function Tests (KFTs)

Mnemonic: "CUBAGE" - to remember all KFT components:

Letter Test
C Creatinine (serum) / Creatinine clearance
U Urea / BUN (Blood Urea Nitrogen)
B Blood electrolytes (Na⁺, K⁺, HCO₃⁻, Cl⁻)
A Albumin / Acid-base status (pH, bicarbonate)
G GFR (eGFR - estimated glomerular filtration rate)
E Electrolytes + Examination of urine (urinalysis, casts, protein)

Letter	Test
C	Creatinine (serum) / Creatinine clearance
U	Urea / BUN (Blood Urea Nitrogen)
B	Blood electrolytes (Na⁺, K⁺, HCO₃⁻, Cl⁻)
A	Albumin / Acid-base status (pH, bicarbonate)
G	GFR (eGFR - estimated glomerular filtration rate)
E	Electrolytes + Examination of urine (urinalysis, casts, protein)

Overview

KFTs assess glomerular filtration, tubular function, and urinary composition. The kidney has enormous reserve capacity - serum creatinine may not rise meaningfully until 50-75% of nephron function is already lost (see graph below).

Nonlinear relationship between GFR and serum creatinine - large drops in GFR cause only modest creatinine rise until function is severely reduced

Key concept: A 75% reduction in GFR (from 120 to 30 mL/min) corresponds to only a modest rise in serum creatinine. This is why creatinine is a late marker of kidney injury.

Tests Grouped by Category

1. Glomerular Filtration Tests

Serum Creatinine

End product of muscle creatine phosphate metabolism
Freely filtered; minimally secreted by tubules
Normal: Males ~0.7-1.3 mg/dL; Females ~0.5-1.1 mg/dL
Limitations: Confounded by muscle mass, age, sex, drugs (e.g. trimethoprim blocks tubular secretion), fluid status, and laboratory technique
Not renal-specific alone - must interpret with clinical context

eGFR (Estimated GFR)

Calculated using CKD-EPI or MDRD equations from serum creatinine, age, sex, race
The single best overall marker of kidney function in clinical practice
Normal: 100-120 mL/min/1.73 m²

GFR Stage	eGFR (mL/min)	Interpretation
Normal	>90	With markers of kidney damage
Mildly reduced	60-89
Mildly-moderately reduced	45-59	CKD G3a
Moderately-severely reduced	30-44	CKD G3b
Severely reduced	15-29	CKD G4
Kidney failure	<15	ESRD - consider dialysis

Creatinine Clearance (CrCl)

24-hour urine collection method
Formula: CrCl = (U × V) / P where U = urine creatinine, V = urine volume (mL/min), P = plasma creatinine
Slightly overestimates GFR (due to tubular secretion of creatinine)
Normal: 100-120 mL/min

2. BUN / Blood Urea Nitrogen

Urea is the main end product of protein catabolism (urea cycle in liver)
Normal: 5-20 mg/dL (1.8-7.2 mmol/L)
Not purely renal - confounded by:
- Protein intake (high protein diet raises BUN)
- Liver disease (impaired synthesis lowers BUN)
- GI bleed, catabolic states, dehydration (raise BUN disproportionately)

BUN:Creatinine Ratio - Clinically Vital

Ratio	Interpretation
>20:1	Prerenal azotemia (dehydration, GI bleed, heart failure)
10-20:1	Normal / Intrinsic renal disease
<10:1	Low protein intake, liver failure, SIADH

Mnemonic: "Twenty-to-One = Pre-Renal Run" - BUN:Cr >20 = prerenal

3. Electrolytes

Electrolyte	Normal	Significance in Renal Disease
Sodium (Na⁺)	135-145 mEq/L	Hyponatremia in SIADH, dilutional states; altered in CKD
Potassium (K⁺)	3.5-5.0 mEq/L	Hyperkalemia is a cardinal feature of AKI/CKD - life-threatening
Bicarbonate (HCO₃⁻)	22-26 mEq/L	Low in metabolic acidosis from CKD (failure to excrete H⁺)
Chloride (Cl⁻)	98-106 mEq/L	Raised in hyperchloremic metabolic acidosis
Phosphate	2.5-4.5 mg/dL	Elevated in CKD (kidneys fail to excrete phosphate)
Calcium	8.5-10.5 mg/dL	Low in CKD (reduced vitamin D activation)
Uric Acid	2.4-7.0 mg/dL (M)	Elevated in gout, CKD; low in Fanconi syndrome

4. Urinalysis (Urine Examination)

Dipstick Analysis

Parameter	Normal	Abnormal Finding	Significance
Protein	Negative (<150 mg/day)	Positive	Glomerular damage, nephrotic syndrome
Blood/Hematuria	Negative	Positive	GN, stones, UTI, malignancy
Glucose	Negative	Positive with normal blood sugar	Tubular dysfunction (Fanconi syndrome)
Nitrites	Negative	Positive	Bacterial UTI
Leukocyte esterase	Negative	Positive	Pyuria, UTI, AIN
pH	4.5-8.0	Alkaline in RTA	Renal tubular acidosis
Specific Gravity	1.001-1.035	Fixed at 1.010	Isosthenuria = tubular damage (AKI)
Ketones	Negative	Positive	DKA, starvation

Urine Microscopy - Casts and What They Mean

Urinary sediments in AKI: A=epithelial cell aggregate, B=hyaline cast (prerenal), C=epithelial cast (early ATN), D=muddy brown cast (established ATN), E=erythrocyte cast (glomerulonephritis), F-G=indinavir crystals

Mnemonic for casts: "Hyaline = Fine; RBC = Glomerulonephritis; WBC = interstitial; Muddy Brown = ATN; Broad Waxy = CKD"

Cast Type	Diagnosis
Hyaline casts	Normal or prerenal AKI (dehydration, mild)
RBC (red cell) casts	Glomerulonephritis, vasculitis (GOLD STANDARD finding)
WBC (white cell) casts	Acute interstitial nephritis (AIN), pyelonephritis
Granular "muddy brown" casts	Acute tubular necrosis (ATN) - >10/LPF = 100% PPV for ATN
Fatty casts / oval fat bodies	Nephrotic syndrome (heavy proteinuria)
Broad / waxy casts	Chronic kidney disease (dilated tubules)
Epithelial cell casts	Early ATN

5. Proteinuria Quantification

Amount	Category	Significance
<150 mg/day	Normal
150-3500 mg/day	Non-nephrotic	CKD, hypertensive nephropathy
>3.5 g/day	Nephrotic range	Minimal change, FSGS, membranous nephropathy, diabetic nephropathy

Spot urine protein:creatinine ratio (UPCR): a ratio of >3.5 = nephrotic range (equivalent to 24h protein >3.5 g)

Types of proteinuria (remember "GOTS"):

Glomerular - filtered proteins (albumin predominant; failure of filtration barrier)
Overflow - excess production saturates reabsorption (Bence-Jones proteins in myeloma)
Tubular - failed reabsorption of normally filtered small proteins
Secretory/tissue - inflammation (Tamm-Horsfall protein)

6. Fractional Excretion of Sodium (FENa)

Used to distinguish prerenal AKI from ATN:

$$\text{FENa} = \frac{U_{Na} / S_{Na}}{U_{Cr} / S_{Cr}} \times 100%$$

FENa	Interpretation
<1%	Prerenal AKI (tubules intact, avidly reabsorbing Na)
>3%	ATN (tubular damage, can't reabsorb Na)
1-3%	Indeterminate

Caveat: FENa <1% can occur in ATN with contrast, myoglobin, sepsis, or heart failure. If patient is on diuretics, use FEUrea <35% = prerenal instead.

Identifying Abnormalities: Disease Patterns

AKI vs CKD - First Differentiation

Feature	AKI	CKD
Onset	Sudden (hours-days)	Gradual (months-years)
Urine output	Often oliguria	Normal initially
Kidney size on US	Normal or enlarged	Small, echogenic
Anaemia	Absent early	Present (low EPO)
Broad waxy casts	Absent	Present
Calcium/Phosphate	Usually normal initially	Hyperphosphatemia, hypocalcemia
Prior creatinine	Normal	Chronically elevated

KDIGO Classification of AKI

Stage	Serum Creatinine Criteria	Urine Output
1	Rise ≥0.3 mg/dL within 48h OR 1.5-1.9× baseline	<0.5 mL/kg/h for 6-12h
2	2.0-2.9× baseline	<0.5 mL/kg/h for ≥12h
3	≥3× baseline OR ≥4.0 mg/dL OR dialysis	<0.3 mL/kg/h for ≥24h or anuria ≥12h

Distinguishing Prerenal, Intrinsic, and Postrenal AKI

Parameter	Prerenal	Intrinsic (ATN)	Postrenal
BUN:Cr ratio	>20:1	10-15:1	Variable
FENa	<1%	>3%	Variable
Urine Na	<20 mEq/L	>40 mEq/L	Variable
Urine osmolality	>500 mOsm/kg	~300 (isosthenuria)	Variable
Urine specific gravity	>1.020	~1.010	Variable
Urine casts	Hyaline (or none)	Muddy brown granular	None
Response to fluids	Improves	No improvement	Improves with relief

Mnemonic: "Prerenal LOVES sodium - holds it tight (FENa <1%, UNa <20)" "ATN LOSES sodium - can't hold it (FENa >3%, UNa >40)"

Nephrotic vs Nephritic Syndrome

Feature	Nephrotic	Nephritic
Proteinuria	>3.5 g/day	<3.5 g/day
Haematuria	Absent/mild	Present (RBC casts)
Hypertension	Mild/absent	Prominent
Oedema	Severe (anasarca)	Mild-moderate
GFR	Near normal early	Reduced
Serum albumin	Low	Normal/mildly low
Serum cholesterol	High	Normal
Urine casts	Fatty, oval fat bodies	RBC casts

Mnemonic - Nephrotic = "PALE": Protein (massive), Albumin (low), Lipids (high), Edema (severe) Mnemonic - Nephritic = "PHARH": Protein (mild), Haematuria, Azotemia, RBC casts, Hypertension

Reference Ranges Summary

Test	Normal Value
Serum Creatinine (M/F)	0.7-1.3 / 0.5-1.1 mg/dL
BUN	5-20 mg/dL
BUN:Creatinine ratio	10-20:1
eGFR	>90 mL/min/1.73m² (normal)
Creatinine Clearance	100-120 mL/min
Urine Protein	<150 mg/day
Urine specific gravity	1.001-1.035
Serum K⁺	3.5-5.0 mEq/L
Serum Na⁺	135-145 mEq/L
Serum HCO₃⁻	22-26 mEq/L
FENa (prerenal)	<1%
FENa (ATN)	>3%

Sources: Comprehensive Clinical Nephrology, 7th Ed.; Brenner and Rector's The Kidney; Morgan and Mikhail's Clinical Anesthesiology, 7th Ed.; Barash Clinical Anesthesia, 9th Ed.; Harper's Illustrated Biochemistry, 32nd Ed.

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