What are serum protein electrophoresis (SPEP), serum immunoelectrophoresis, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), parathyroid hormone (PTH), calcium, and phosphate used for?

Clinical Uses of SPEP, Serum Immunoelectrophoresis, CRP, ESR, PTH, Calcium, and Phosphorus

These laboratory tests serve distinct diagnostic purposes: SPEP and serum immunoelectrophoresis identify monoclonal gammopathies (particularly multiple myeloma); CRP and ESR measure systemic inflammation; and PTH, calcium, and phosphorus evaluate calcium homeostasis and parathyroid disorders.

Serum Protein Electrophoresis (SPEP) and Serum Immunoelectrophoresis

Primary Clinical Application

SPEP is the primary screening tool for detecting monoclonal proteins (M-proteins) in plasma cell disorders, particularly multiple myeloma, Waldenström's macroglobulinemia, and monoclonal gammopathy of undetermined significance 1, 2.

• Monoclonal gammopathies produce a characteristic homogeneous spike-like peak in the gamma-globulin region, indicating a clonal malignant or potentially malignant process 2
• Polyclonal gammopathies show diffuse elevation and indicate reactive or inflammatory processes 2

Diagnostic Workflow

When SPEP shows an M-protein spike:

• Serum immunofixation electrophoresis (SIFE) identifies the specific immunoglobulin type (IgG, IgA, IgM, IgD, IgE) and light chain (kappa or lambda) 1
• Quantification of M-protein helps differentiate multiple myeloma from other monoclonal gammopathies 1
• Serial SPEP measurements track disease progression and treatment response 1

Important Caveats

• SPEP has significant limitations at low M-protein concentrations (<1 g/L), with decreased accuracy and precision 3
• Background gamma globulin levels affect quantification accuracy—hypergammaglobulinemia causes overestimation, particularly with the perpendicular drop gating method 3
• Always use the same laboratory and method for serial monitoring to ensure accurate comparison 1

C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR)

Clinical Application

Both CRP and ESR are nonspecific markers of systemic inflammation, but they should not be routinely ordered together as CRP is generally superior 4, 5.

Key Differences

• CRP responds more rapidly to acute inflammation (rises within 6-8 hours, peaks at 48 hours) and is less affected by age, anemia, and plasma proteins 4
• ESR rises more slowly, is influenced by age, sex, anemia, plasma proteins, and red blood cell morphology 4, 6

When to Use Each Test

Use CRP preferentially for:

• Acute inflammatory conditions
• Monitoring treatment response (faster normalization)
• Patients with obesity (BMI is the main determinant of CRP elevation in non-inflammatory states) 7

ESR may be preferred for:

• Temporal arteritis/giant cell arteritis (where very high ESR >100 mm/hr has diagnostic significance)
• Conditions where historical ESR trends exist

Common Pitfalls

• Discordance between CRP and ESR is common 4, 5:
  • High ESR with normal CRP: typically associated with advanced age 7
  • High CRP with normal ESR: typically associated with elevated BMI and obesity 7
• Neither test has adequate sensitivity or specificity for standalone diagnosis—always interpret with clinical context 4
• Co-ordering both tests is wasteful and provides minimal additional clinical value 5

PTH, Calcium, and Phosphorus

Primary Clinical Applications

This triad evaluates calcium homeostasis disorders, particularly hyperparathyroidism and hypoparathyroidism 8, 9.

Diagnostic Algorithm for Calcium Disorders

For Hypercalcemia:

1. Measure serum calcium, PTH, and phosphorus simultaneously
2. Elevated or inappropriately normal PTH with hypercalcemia = primary hyperparathyroidism 8
3. Suppressed PTH with hypercalcemia = PTH-independent causes (malignancy, vitamin D toxicity, granulomatous disease) 8

For Hypocalcemia:

1. Measure serum calcium, PTH, phosphorus, and magnesium
2. Low/inappropriately normal PTH with hypocalcemia = hypoparathyroidism 9
3. Elevated PTH with hypocalcemia = secondary hyperparathyroidism (vitamin D deficiency, chronic kidney disease, malabsorption) 8
4. Check magnesium—severe hypomagnesemia impairs PTH secretion and mimics hypoparathyroidism 9

Specific Clinical Scenarios

Chronic Kidney Disease:

• PTH monitoring detects secondary hyperparathyroidism and metabolic bone disease 8
• C-terminal PTH fragments accumulate with declining renal function, causing assay variability 8
• Guidelines recommend treating when PTH exceeds 2-9 times the upper reference limit 8

Post-Bariatric Surgery:

• Monitor PTH, calcium, and phosphorus regularly to detect secondary hyperparathyroidism from malabsorption 8
• Prevalence of secondary hyperparathyroidism reaches 63% at 5 years post-surgery 8

Perioperative Parathyroid/Thyroid Surgery:

• Intraoperative PTH measurement confirms successful parathyroid adenoma removal 8
• Third-generation PTH assays reflect treatment success more rapidly than second-generation assays 8

Critical Pitfalls with PTH Measurement

• PTH assays are NOT standardized—different laboratories and assay generations produce significantly different results 8
• Second-generation assays (most common) measure intact PTH plus some fragments
• Third-generation assays measure only PTH(1-84)
• Always use the same laboratory and assay for serial monitoring 8
• PTH is influenced by: race (higher in Black individuals), age (increases with age), BMI (higher in obesity), vitamin D status, and biotin supplements 8
• Optimal specimen: EDTA plasma at 4°C (more stable than serum at room temperature) 8

Phosphorus Interpretation

• Hyperphosphatemia with hypocalcemia = hypoparathyroidism or chronic kidney disease 9
• Hypophosphatemia with hypercalcemia = primary hyperparathyroidism 8
• Elevated calcium-phosphate product (>55 mg²/dL²) increases risk of ectopic calcification 9