Friday, February 19, 2010

Mind the Gap

A 65 year old Caucasian man presents with bone pain in his back and chest. His primary care doctor ruled out coronary causes and referred him to you. The pain is induced by movement but does not wake him from sleep. His past medical history is significant for COPD, PTSD, and atherosclerosis. He takes a baby aspirin. His family history is noncontributory. His social history is significant for smoking 50 pack-years and drinking a beer a night. He is married, has two children, and is a retired auto mechanic. Review of systems is positive for fatigue and generalized weakness.

The laboratory values are: sodium 135, potassium 4, chloride 110, bicarbonate 24, BUN 10, Cr 1.6, glucose 95. Calcium, magnesium, phosphorus, and albumin are normal. The patient has a normocytic, normochronic anemia of Hgb 11 g/dL. WBC and platelets are normal. An X-ray is shown below.

Challenge: What's the diagnosis?

Image is shown under Fair Use.


tree said...

multiple myeloma

Suheil said...

Osteomalacia? Looser's zone on the X-ray?

Craig Chen said...

multiple myeloma!
Mind the Gap

The anion gap is calculated by subtracting measured anions from measured cations (Na - HCO3 - Cl). The anion gap here is low. The differential diagnosis is laboratory error, hypoalbuminemia, hyperkalemia, hypercalcemia, hypermagnesemia, lithium intoxication, or a cationic paraprotein such as multiple myeloma. Interestingly, a negative anion gap is seen in hypernatremia (underestimated by the machine), hyperlipidemia (due to light scattering in the colorimetric assay overestimating chloride), or bromide intoxication.

In this case, the decreased anion gap is due to multiple myeloma creating unmeasured cations. Multiple myeloma is a proliferation of a single clone of plasma cells producing monoclonal immunoglobulin. The X-ray shows lytic lesions without reactive sclerosis of the pelvis and proximal femora with a "soap-bubble" appearance in the ischia. The renal disease also suggests multiple myeloma.

Source: UpToDate;