Examkin

Question

Difficulty: HardInterpreting Laboratory Results

A 68-year-old man presents to the clinic complaining of progressive fatigue, constipation, and lower back pain for the past 3 months. He has no prior medical history and takes no medications. Physical examination is notable for mucosal pallor and point tenderness over the L3 and L4 vertebrae.

Laboratory evaluation reveals the following:

Laboratory TestResultReference Range
Serum Chemistry
Sodium136 mEq/L136-145 mEq/L
Potassium4.2 mEq/L3.5-5.0 mEq/L
Chloride112 mEq/L98-106 mEq/L
Bicarbonate22 mEq/L22-28 mEq/L
Blood urea nitrogen45 mg/dL8-20 mg/dL
Creatinine2.4 mg/dL0.7-1.3 mg/dL
Calcium12.1 mg/dL8.4-10.2 mg/dL
Total protein10.8 g/dL6.0-7.8 g/dL
Albumin3.4 g/dL3.5-5.5 g/dL
Complete Blood Count
Hemoglobin9.2 g/dL13.5-17.5 g/dL
Mean corpuscular volume88 fL80-100 fL
Platelets180,000/mm³150,000-400,000/mm³
Leukocytes5,400/mm³4,500-11,000/mm³

Based on the synthesis of these laboratory findings, which of the following is the most likely diagnosis?

  1. A
    Primary hyperparathyroidism
  2. B
    Sarcoidosis
  3. Multiple myelomaAnswer
  4. D
    Waldenström macroglobulinemia
  5. E
    Metastatic prostate adenocarcinoma

Answer

Multiple myeloma
The correct diagnosis is multiple myeloma. This patient's presentation of fatigue, constipation (a symptom of hypercalcemia), and back pain suggests a malignant process. The laboratory results reveal hypercalcemia, renal insufficiency, and normocytic anemia, which constitute the 'CRA' of the CRAB criteria for multiple myeloma. Furthermore, calculation of the serum anion gap (Na+−[Cl−+HCO3−]Na^+ - [Cl^- + HCO_3^-]) yields a value of 2 mEq/L (normal 8-12 mEq/L). A narrow or negative anion gap is a classic laboratory artifact caused by high levels of cationic (positively charged) paraproteins, such as IgG monoclonal antibodies, which increase the unmeasured cations in the serum. The presence of these paraproteins is corroborated by the markedly elevated globulin gap (Total protein - Albumin = 7.47.4 g/dL). The synthesis of a narrow anion gap, elevated globulin gap, hypercalcemia, renal failure, and anemia makes multiple myeloma the definitive diagnosis.

Step-by-Step Solution

1
Analyze the basic metabolic panel and complete blood count for patterns of organ dysfunction.
The patient exhibits hypercalcemia (12.1 mg/dL), acute or chronic kidney injury (creatinine 2.4 mg/dL), and a normocytic anemia (hemoglobin 9.2 g/dL), fulfilling the 'CRA' of the CRAB criteria.
Recognizing the constellation of hypercalcemia, renal failure, and anemia in an elderly patient with back pain strongly suggests a malignant plasma cell dyscrasia.
2
Calculate the globulin gap to investigate the markedly elevated total protein.
Globulin gap = Total protein - Albumin = 10.8−3.4=7.410.8 - 3.4 = 7.4 g/dL.
A normal globulin gap is <4< 4 g/dL. A gap >4> 4 g/dL indicates a significant increase in non-albumin serum proteins, most commonly due to monoclonal or polyclonal hypergammaglobulinemia.
3
Calculate the serum anion gap to evaluate the acid-base and electrolyte status.
Anion Gap = Na+−(Cl−+HCO3−)=136−(112+22)=2Na^+ - (Cl^- + HCO_3^-) = 136 - (112 + 22) = 2 mEq/L.
The normal anion gap is roughly 8-12 mEq/L. A noticeably narrow or negative anion gap is a classic artifact caused by high serum concentrations of cationic (positively charged) paraproteins, typical of an IgG monoclonal gammopathy. This conclusively ties the protein elevation to the electrolyte abnormality.

Key Concept

Interpretation of derived laboratory indices (anion gap and globulin gap) to diagnose paraproteinemias
Estimated Time:1m 30s
Rate this question

Topics

Interpreting Laboratory Results

More questions from this topic