# RENAL LABS AND IMAGING: ESSENTIAL DIAGNOSTIC GUIDE

## PA/Medical Student Handout

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## LEARNING OBJECTIVES

By the end of this module, you will be able to:

1.  **Order appropriate labs** based on clinical presentation (new patient vs. follow-up)
2.  **Understand renal function assessment** including eGFR, cystatin C, and creatinine limitations
3.  **Interpret laboratory panels** for specific disease states (CKD, HTN, GN)
4.  **Select optimal imaging modality** for common clinical scenarios
5.  **Understand radiation exposure** and contrast considerations
6.  **Interpret imaging findings** and recognize abnormalities
7.  **Manage contrast-related risks** in patients with reduced kidney function

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## SECTION 1: ESSENTIAL RENAL LABS

### Lab Panels by Clinical Scenario

#### **Panel 1: New Nephrotic Syndrome Patient**

**Order**: RFP, CBC, PTH, Mg, Uric Acid, Monoclonal Screen (SPEP), UA with Micro, Albumin-to-Creatinine Ratio (ACR), Protein-to-Creatinine Ratio (PCR)

| Lab | Why Order? | What It Tells You |
|----|----|----|
| **RFP** | Renal function baseline | Creatinine, BUN, eGFR |
| **CBC** | Hematologic status | Anemia (hemolysis? bleeding?) |
| **PTH** | Mineral metabolism | Parathyroid status; renal osteodystrophy risk |
| **Mg** | Electrolyte screening | Hypomagnesemia common in proteinuria |
| **Uric Acid** | Gout risk; cell turnover | Can precipitate AKI during diuresis |
| **SPEP** | Screen for myeloma | Monoclonal protein? Light chain disease? |
| **UA with Micro** | Urine pathology | RBC casts? (GN) WBC casts? (infection) |
| **ACR** | Quantify proteinuria | Better than dipstick; albumin-specific |
| **PCR** | Total proteinuria | Includes non-albumin proteins |

#### **Panel 2: Follow-Up CKD Patient (Stable)**

**Order**: RFP, CBC, PTH, Mg, Uric Acid, ACR

**Frequency**: Q3-6 months if stable; Q1-2 months if declining

#### **Panel 3: Hypertension Workup**

**Order**: Cortisol (24-hour or late-night salivary), TSH, PTH, Plasma Free Metanephrines, Aldosterone-to-Renin Ratio

| Test | When to Order | What It Identifies |
|----|----|----|
| **24-hr Cortisol** | Suspect Cushing’s | Cortisol excess (HTN + weight gain + striae) |
| **TSH** | Screen for thyroid | Hypothyroidism can cause HTN |
| **PTH** | Screen for hypercalcemia | Hyperparathyroidism (HTN + hypercalcemia) |
| **Plasma Metanephrines** | Suspect pheochromocytoma | Catecholamine excess (episodic HTN, sweating) |
| **Aldo-Renin Ratio** | Suspect primary aldosteronism | Hypokalemia + HTN (resistant to therapy) |

#### **Panel 4: Glomerulonephritis Workup**

**Order**: C3, C4, Hepatitis Panel, ANA, ANCA, Anti-GBM Antibody, Anti-dsDNA Antibody, Anti-PLA2R Antibody

| Antibody Test | Positive In | Clinical Significance |
|----|----|----|
| **C3, C4** | Lupus, membranoproliferative GN | Complement consumption; disease activity |
| **ANA** | Lupus; must-have for SLE diagnosis | ANA-negative SLE exists but rare (\<5%) |
| **Anti-dsDNA** | Lupus (specific); lupus nephritis | High titers = lupus activity |
| **ANCA** | ANCA-associated vasculitis | PR3-ANCA (granulomatosis) vs. MPO-ANCA (microscopic polyangiitis) |
| **Anti-GBM** | Anti-GBM disease (Goodpasture) | Pulmonary-renal syndrome; urgent need for plasmapheresis |
| **Anti-PLA2R** | Membranous nephropathy | 70-80% of cases; marker of disease remission |
| **Hepatitis Panel** | Screen before immunosuppression | Critical before starting azathioprine, mycophenolate |

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## SECTION 2: UNDERSTANDING RENAL FUNCTION

### The eGFR Equation

**CKDEPI 2021 Equation** (currently recommended):

    eGFR = 142 × (Serum Creatinine / λ)^α × (0.9938)^age × (0.7 if female)

Where: - λ = 0.7 (females), 0.9 (males) - α = -0.241 (females), -0.302 (males)

**Key Points**: - eGFR predicts **GFR-based kidney function** (not tubular function) - eGFR becomes **less accurate** if: - Rapidly changing creatinine (AKI) - Extreme muscle mass (bodybuilders, amputees) - Liver cirrhosis - Pregnancy - Vegetarian diet (less creatinine production)

### Cystatin C: Alternative Marker

**Advantages of Cystatin C**: - Produced at constant rate (independent of muscle mass) - Better in elderly, obese, or cachetic patients - Better predictor of cardiovascular events

**Disadvantages**: - More expensive than creatinine - Less standardized across labs - Still affected by inflammatory states, corticosteroid use

**When to Use Cystatin C**: - Borderline eGFR (45-60) for decision-making - Extreme body habitus - Elderly/frail patients with low muscle mass - When creatinine unreliable

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## SECTION 3: RENAL IMAGING MODALITIES

### Quick Reference: Which Imaging for What?

| Clinical Scenario | BEST Modality | Why | Radiation | Contrast |
|----|----|----|----|----|
| **Suspected stone** | Non-contrast CT (gold standard) | 98% sensitivity; fast | Yes (low) | NO |
| **Hydronephrosis?** | Renal ultrasound | Radiation-free; real-time | No | NO |
| **Renal mass** | CT with contrast (multiphasic) | Enhancement characterization | Yes | YES (iodine) |
| **RAS (high suspicion)** | CT angiography | 94-100% sensitivity | Yes | YES (iodine) |
| **RAS (screening)** | Duplex ultrasound | Low sensitivity but no radiation | No | NO |
| **Scarring from pyelonephritis** | DMSA scintigraphy | Gold standard for cortical pathology | Yes (nuclear) | NO |
| **Obstruction vs non-obstruction?** | Diuretic renogram | Functional assessment; F+20 protocol | Yes (nuclear) | NO |
| **Detailed mass characterization** | MRI with contrast | Best soft tissue; NO radiation | NO | YES (gadolinium) |
| **Renal trauma?** | CT with contrast | Full organ assessment | Yes | YES (iodine) |
| **Transplant function** | Duplex ultrasound + nuclear imaging | Perfusion + function | No/Yes | NO |

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### COMPUTED TOMOGRAPHY

#### Non-Contrast CT (Best for Stones)

**Indications**: - Suspected urolithiasis - Renal size/contour - Hydronephrosis assessment - Post-procedural complications

**Advantages**: - 98% sensitivity, 96-100% specificity for stones - NO contrast risk - Fast; can detect non-calcified pathology (blood, perfusion changes) - Low radiation dose achievable with modern protocols

**Disadvantages**: - Cannot characterize renal masses (no enhancement pattern) - Cannot assess renal function - Radiation exposure

#### Contrast-Enhanced CT (for Masses and Vascular)

**Phases of Enhancement**: 1. **Arterial phase** (25-30 sec): Renal artery, hypervascular lesions 2. **Nephrographic phase** (80-100 sec): Peak parenchymal enhancement 3. **Delayed phase** (8-15 min): Collecting system, excretion

**Clinical Uses**: - Renal mass characterization (Bosniak classification for cysts) - CT angiography for RAS - Pyelonephritis, renal infarction - Transplant evaluation

**Radiation Dose**: 3-7 mSv (equivalent to 3 years background radiation)

#### CT Urography

**Combines**: - Non-contrast phase (baseline, stones, blood) - Nephrographic phase (mass detection) - Delayed excretory phase (collecting system, ureters, bladder)

**Best for**: Hematuria evaluation; detailed urinary tract assessment

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### RENAL ULTRASONOGRAPHY

**Advantages**: - NO radiation - Real-time imaging (can assess dynamics) - Bedside availability (POCUS) - Good for fluid assessment - Ideal in pregnancy, pediatrics

**Disadvantages**: - Operator-dependent (60-90% sensitivity range) - Reduced accuracy in obesity - Cannot assess contrast enhancement - Limited specificity for mass characterization

**Key Measurements**: - **Renal length**: Normal 9-13 cm (\>9 cm preferred for biopsy) - **Cortical thickness**: Measured at level of hilum (normal \>7-8 mm) - **Collecting system dilatation**: Anteroposterior pelvic diameter \>10 mm suggests hydronephrosis (in context)

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### RENAL ARTERY DUPLEX ULTRASONOGRAPHY

**For Renovascular Hypertension Screening**:

**Diagnostic Criteria**: - **Peak Systolic Velocity (PSV)** \>200 cm/s = significant stenosis (≥60%) - **Renal-Aortic Ratio (RAR)** \>3.5 = additional evidence of stenosis - **End-Diastolic Velocity** \>70 cm/s = hemodynamically significant

**Sensitivity**: 85-92% (in experienced hands) **Specificity**: 85-95%

**Limitations**: - Operator-dependent (learning curve: hundreds of exams) - Fails in 20-30% of obese patients - Cannot visualize accessory renal arteries reliably - Technical failure high in some institutions

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### NUCLEAR MEDICINE IMAGING

#### Diuretic Renogram (Lasix Renogram)

**What It Does**: Assesses **function** (not anatomy)

**Protocol**: 1. Give Tc-99m MAG3 radiopharmaceutical 2. Acquire images for 20 minutes (uptake phase) 3. Give furosemide at 20-minute mark (F+20 protocol) 4. Continue imaging 20 more minutes

**Interpretation**: - **Time-to-Peak (TTP)**: When renal activity peaks (normal \<3 min) - **Half-Time (T1/2)** post-diuretic: Time for activity to drop 50% (normal \<10 min) - **T1/2 \>20 min** = obstructive pattern - **T1/2 10-20 min** = equivocal (may need repeat/further imaging)

**Best For**: Distinguishing obstructive from non-obstructive hydronephrosis

#### DMSA Scintigraphy

**What It Does**: Images **kidney cortex** (high-resolution scarring assessment)

**Uses**: - Acute pyelonephritis (hypoperfusion areas) - Chronic scarring (from reflux or infection) - Differential renal function assessment - Pediatric pyelonephritis/reflux evaluation

**Sensitivity**: \>95% for detecting scars \>1 cm

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## SECTION 4: CONTRAST CONSIDERATIONS

### Iodinated Contrast (Used in CT)

#### From “Contrast-Induced Nephropathy” to “Contrast-Associated Nephropathy”

**Paradigm Shift**: - Old term: **Contrast-Induced Nephropathy (CIN)** - implied direct causal relationship - New term: **Contrast-Associated Nephropathy (CAN)** - acknowledges other factors - Current: **Contrast-Induced Acute Kidney Injury (CI-AKI)** when probable causal relationship

**Key Insight**: Many post-contrast creatinine elevations are due to **other factors** (illness severity, dehydration, other medications), not the contrast itself.

#### Risk Stratification

**Low Risk** (Creatinine elevation \<2%): - Normal renal function (eGFR \>60) - No diabetes

**Intermediate Risk** (Creatinine elevation 5-10%): - eGFR 45-60 with diabetes OR eGFR 30-45 without diabetes - Age \>75 - Heart failure

**High Risk** (Creatinine elevation 15-30%): - eGFR \<30 - Diabetes + CKD - Multiple risk factors

**Prevention**: - **IV hydration** with normal saline (best evidence) - Sodium bicarbonate may help (controversial) - Metformin: Hold if eGFR \<30; resume 48 hours post-contrast - NSAIDs: Hold 48 hours before and after - ACE-I/ARB: Can hold 24 hours before (optional)

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### Gadolinium-Based Contrast (Used in MRI)

#### The Nephrogenic Systemic Fibrosis (NSF) Risk

**Group Classification** (Risk of NSF):

| Group | Agents | NSF Risk | Recommendation |
|----|----|----|----|
| **I (Highest)** | Gadodiamide (Omniscan), Gadopentetate (Magnevist), Gadoversetamide (OptiMARK) | Higher risk | **CONTRAINDICATED** in eGFR \<30 |
| **II (Lowest)** | Gadoteridol (ProHance), Gadoterate (Dotarem), Gadobutrol (Gadavist) | Minimal/NO NSF cases | Can use in eGFR \<30; prefer these |
| **III (Intermediate)** | Others | Unknown | Use cautiously; prefer Group II |

#### Current Guidelines

**NEW (2024)**: Group II GBCAs can be used even in advanced CKD (eGFR \<30) or dialysis patients because: - No unconfounded NSF cases reported with Group II agents - Risk of not getting diagnostic imaging may outweigh minimal NSF risk - Use lowest effective dose

**Practical Approach**: - Check eGFR before MRI - If eGFR \<30: Use **Group II GBCA only** (ProHance, Dotarem, Gadavist) - If eGFR \>30: Any group acceptable (Group II still preferred) - Dialysis patients: Can receive Group II GBCA; no need to time dialysis

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## SECTION 5: CLINICAL INTERPRETATION

### Reading a Renal CT Report

**Standard Elements**: 1. **Kidney size**: Length in cm (normal 9-13 cm) 2. **Cortical thickness**: Measured at hilum (normal \>7 mm) 3. **Enhancement pattern**: Symmetric? Hypodense areas? 4. **Collecting system**: Dilated? Hydronephrosis present? 5. **Vascular**: Assess for stenosis, accessory arteries 6. **Retroperitoneum**: Free fluid? Mass? Fat stranding?

**Red Flags**: - Asymmetric renal atrophy → suggests chronic vascular disease or scarring - Stranding around kidney → infection, infarction, or hemorrhage - Enlarged kidney → acute obstruction, acute infection, or infiltration

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## SECTION 6: CLINICAL PEARLS

✓ **Non-contrast CT is gold standard** for suspected stones (98% sensitivity)

✓ **CT angiography superior to duplex** for diagnosing RAS (94-100% vs. 85-95%)

✓ **eGFR accurate when stable** but unreliable in AKI or extreme body habitus

✓ **Cystatin C useful** as complementary marker in borderline eGFR

✓ **Ultrasound BEST for** hydronephrosis screening, real-time assessment, pregnant patients

✓ **Nuclear imaging best for FUNCTION** (diuretic renogram for obstruction; DMSA for scars)

✓ **Group II gadolinium has a very low NSF risk** even in advanced CKD (risk is very low, not proven zero; balance against delayed diagnosis)

✓ **IV hydration prevents** most contrast-related complications; more important than contrast selection

✓ **Post-contrast creatinine elevation** often has OTHER causes; not always “contrast-induced”

✓ **Diuretic renogram (F+20 protocol)** definitively distinguishes obstructive from non-obstructive hydronephrosis

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## PRACTICE QUESTIONS

**Question 1**: A 45-year-old male with CKD Stage 3b (eGFR 38) presents with hematuria. You need to evaluate for renal masses. Which imaging is BEST?

1.  Duplex ultrasound (no radiation)
2.  Non-contrast CT (no iodine contrast)
3.  Contrast-enhanced CT with iodine (multiplane imaging)
4.  MRI with gadolinium

**Answer**: **C** - For renal mass characterization, **contrast-enhanced CT** is superior because: - Iodine contrast allows assessment of enhancement (key for benign vs. malignant) - eGFR 38 = **intermediate risk** for contrast-related AKI (5-10%), but manageable with IV hydration - CT faster and more available than MRI - MRI would be alternative if contrast contraindicated, but is NOT first-line

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**Question 2**: A 72-year-old woman with hypertension refractory to 3 agents has eGFR 42. You want to screen for renovascular hypertension. What is the BEST initial test?

1.  Renal artery duplex ultrasound
2.  Captopril renography
3.  CT angiography
4.  Start on alpha-blocker for “resistant HTN”

**Answer**: **A** - **Duplex ultrasound is appropriate first-line** because: - No radiation, no contrast - Sensitivity 85-92% (good enough for screening) - Low cost - Non-invasive - If positive → confirm with CTA before intervention - If negative + high suspicion → proceed to CTA - Duplex failure rate (20-30% in obese) means know your institution’s capability

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**Question 3**: Labs show eGFR 65 by creatinine, but cystatin C suggests eGFR 52. What is the MOST likely explanation?

1.  Patient has early diabetic kidney disease
2.  Patient is a muscular bodybuilder with high creatinine production
3.  Cystatin C is unreliable; always trust creatinine
4.  Patient needs immediate dialysis referral

**Answer**: **B** - **Discordance between creatinine-based and cystatin C eGFR** suggests: - High muscle mass → high creatinine production → **eGFR overestimated by creatinine** - Cystatin C (muscle-independent) more accurate - Clinical decision: **Trust cystatin C (52)** rather than creatinine (65) - Implication: eGFR 52 = Stage 3b CKD; need closer monitoring/referral

In contrast, low muscle mass (elderly, cachetic) would show **higher cystatin C-based eGFR**, making creatinine-based eGFR the underestimate.

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## KEY TAKEAWAYS

✓ **Lab panels vary** by presentation (new vs. follow-up; suspected HTN vs. GN)

✓ **eGFR useful** for stable patients; unreliable in AKI, extreme body habitus

✓ **Cystatin C complementary** to creatinine; helps identify muscle mass issues

✓ **CT best for anatomy** (stones, masses, vasculature); ultrasound for real-time, radiation-free

✓ **Nuclear imaging best for FUNCTION** (diuretic renogram, DMSA)

✓ **Iodine contrast safe** in CKD with proper hydration; intermediate risk in Stage 3-4

✓ **Gadolinium: very low NSF risk** in advanced CKD if **Group II agent used**

✓ **Ultrasound is best** for pregnancy, pediatrics, real-time assessment

✓ **Duplex ultrasound limited** but appropriate for RAS screening in stable patients

✓ **Diuretic renogram definitive** for distinguishing obstructive vs. non-obstructive hydronephrosis

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## See Also

### Related Student Handouts

- CKD Overview and Classification
- AKI Workup and Diagnosis
- Secondary Hypertension (including RAS)
- Nephrolithiasis
- Kidney Biopsy Essentials
- Urinalysis

### Clinical Content (01-Clinical-Medicine/Nephrology)

- Procedures and Diagnostics Hub
- CKD Hub - Full Clinical Reference
- AKI Hub - Full Clinical Reference
- Hypertension Management Hub

### Butler-COM Resources

- Butler COM - Nephrology Deep Dive

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## RELATED CLINICAL NOTES

- CKD Staging and Management
- AKI Workup and Recovery
- RAS Diagnosis and Treatment
- Stone Disease Management

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*This handout is designed for PA/medical student education. Always consult current institutional protocols for imaging and lab ordering.*


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