# Chapter 3: Laboratory Assessment

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### 📖 Ready for a Deeper Dive?

Explore these related Lecture Series modules and Clinical Mastery reviews:

[BMP Overview & Approach](https://urinenephrology.org/2025_UDPA_Lectures_Live/electrolytes/bmp-overview.html) [Creatinine & GFR](https://urinenephrology.org/2025_UDPA_Lectures_Live/electrolytes/creatinine-gfr.html)

## Basic Renal Function & Metabolic Tests

A comprehensive assessment of kidney health requires a panel of tests that evaluate filtration, tubular function, and metabolic homeostasis.

### Renal Function Panel (RFP)

**Blood Urea Nitrogen (BUN):** Measures nitrogen waste from protein metabolism. Elevated levels can indicate decreased kidney function, dehydration, or increased protein catabolism. A BUN/Cr ratio \>20:1 suggests a prerenal state.

**Serum creatinine:** A filtration marker influenced by muscle mass, diet, supplements, tubular secretion, and the time course of illness. A rising concentration can reflect reduced filtration or non-GFR effects; interpret the trend with urine findings, urine output, medicines, and clinical context. Standard eGFR assumes a relatively steady state.

**eGFR (estimated Glomerular Filtration Rate):** An estimate of kidney filtering capacity based on creatinine, age, and sex. It is the primary metric for staging CKD.

**Serum Electrolytes (Na, K, Cl, CO2):** Reveals the kidney's ability to maintain fluid and electrolyte balance. Abnormalities can suggest kidney dysfunction or specific tubular disorders.

### Alternative GFR Markers

**Cystatin C:** A protein produced by all nucleated cells, Cystatin C is less affected by muscle mass than creatinine, making it a more accurate GFR marker in certain populations (e.g., elderly, amputees).

## Mineral Metabolism Panel

Essential for diagnosing and managing Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD).

**Serum Calcium:** Evaluates parathyroid function and MBD. Must be corrected for albumin.

**Serum Phosphorus:** Levels typically rise in advanced CKD and contribute to vascular calcification and secondary hyperparathyroidism.

**Magnesium (Mg):** Assesses for electrolyte imbalances that can occur in kidney disease or with certain medications, affecting neuromuscular and cardiac function.

**Intact Parathyroid Hormone (iPTH):** Measures the severity of secondary hyperparathyroidism in CKD, guiding treatment decisions.

**25-hydroxy Vitamin D:** Assesses nutritional status and guides supplementation, as its activation is impaired in kidney disease.

## Urine Studies

### Urinalysis (Dipstick and Microscopic)

A properly performed and interpreted urinalysis is a "liquid biopsy," offering clues to diagnosis, localization of disease, and disease activity. Key findings are detailed in Chapter 4.

### Urine Protein and Albumin Measurements

- **Albumin-to-Creatinine Ratio (ACR):** A spot urine test that specifically measures albumin leakage. It is the most sensitive test for detecting early glomerular damage, especially in diabetic nephropathy. A normal ACR is \<30 mg/g.
- **Protein-to-Creatinine Ratio (PCR):** A spot urine test that measures total protein excretion. It is useful for quantifying and monitoring established proteinuric diseases. A nephrotic-range PCR is \>3.5 g/g.

### Urine Electrolytes & Fractional Excretions

These calculations are crucial in the workup of AKI and electrolyte disorders.

**Fractional Excretion of Sodium (FENa)** = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100

A FENa \<1% suggests a prerenal state. A FENa \>2% suggests tubular damage (ATN).

**Fractional Excretion of Urea (FEUrea)** = (Urine Urea × Serum Cr) / (Serum Urea × Urine Cr) × 100

FEUrea is a useful alternative to FENa in patients on diuretics. An FEUrea \<35% suggests a prerenal cause.

## Immunologic & Hematologic Tests

These tests are ordered when there is suspicion for glomerular disease, vasculitis, or a paraprotein-related process.

### Monoclonal Protein Screening

- **Serum/Urine Protein Electrophoresis (SPEP/UPEP):** Identifies and quantifies monoclonal proteins ("M-spikes") in serum or urine.
- **Serum Free Light Chains (FLC):** A highly sensitive assay to detect and monitor monoclonal free light chains (kappa or lambda) that can cause kidney damage.
- **Immunofixation:** Characterizes the specific type of monoclonal protein present (e.g., IgG kappa), providing a definitive diagnosis.

### Autoimmune and Serologic Markers

| Test | Associated Disease(s) | Clinical Utility |
|----|----|----|
| **Anti-Nuclear Antibody (ANA)** | Systemic Lupus Erythematosus (SLE) | Sensitive but not specific screening test for autoimmune diseases. |
| **Anti-dsDNA** | Lupus Nephritis | Highly specific for SLE and correlates with disease activity. |
| **Complement (C3, C4)** | Lupus Nephritis, Post-infectious GN, MPGN | Low levels suggest immune complex-mediated disease. |
| **ANCA (c-ANCA/PR3, p-ANCA/MPO)** | Small Vessel Vasculitis (GPA, MPA) | Key for diagnosing pauci-immune rapidly progressive glomerulonephritis. |
| **Anti-GBM Antibody** | Goodpasture's Disease | Highly specific for anti-glomerular basement membrane disease. |
| **Anti-PLA2R Antibody** | Primary Membranous Nephropathy | Highly specific; can diagnose primary MN without a biopsy in many cases. |

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