Case: A 58-year-old man underwent MRI total body imaging. Later that day, a chemistry profile was ordered. The total calcium was measured at 4 mg/dL (reference interval: 8.5-10.5 mg/dL) and the albumin was 4 g/dL (reference interval: 3.5-5.0 g/dL). The patient was examined and lacked any clinical evidence of hypocalcemia. One week prior to the MRI, the total calcium was normal. What is the explanation for the hypocalcemia?
Answer: Certain gadolinium contrast agents falsely lower the measurement of total calcium when calcium is measured using colorimetric assays. Gadodiamide (Omniscan) and gadoversetamide (OptiMARK) have been reported to cause such "pseudohypocalcemia."  On the other hand, gadoteridol (ProHance) and gadopentetate dimeglumine (Magnevist) have not been reported to effect total calcium measured colorimetrically. In a recent paper from Germany, gadobutrol (Gadovist) and gadoxetate disodium (Primovist/Eovist) were reported to not affect calcium measurements. 
If your hospital uses a gadolinium formulation that does interfere, it is recommended that calcium be measured before the procedure or more than 4 hours post-MRI when the patient's GFR is =>90 mL/min/1.73 M2 or more than 50 hours post-MRI when the GFR is reduced to ~20 mL/min/1.73 M2.
- Emerson J, Kost G. Spurious hypocalcemia after Omniscan- or OptiMARK-enhanced magnetic resonance imaging: an algorithm for minimizing a false-positive laboratory value. Arch Pathol Lab Med. 2004 Oct;128(10):1151-6.
- Löwe A, Breuer J, Palkowitsch P. Evaluation of the effect of two gadolinium-containing contrast-enhancing agents, gadobutrol and gadoxetate disodium, on colorimetric calcium determinations in serum and plasma. Invest Radiol. 2011 Jun;46(6):366-9.
Final question: what disease can develop in patients with renal failure who receive gadolinium for scans? (answer next week).
Gadolinium-associated nephrogenic systemic fibrosis. 
- Schlaudecker JD, Bernheisel CR. Gadolinium-associated nephrogenic systemic fibrosis. Am Fam Physician. 2009 Oct 1;80(7):711-4.