Rank the following laboratory tests and procedures in terms of their relative value to a 65-year-old woman who is to undergo elective resection of a sigmoid cancer

C. Electrocardiogram (ECG).

D. Hemoglobin concentration (Hgb).

F. Arterial blood oxygen tension (PaO 2) on room air.

E. Serum creatinine (Cr).

A. Carcinoembryonic antigen (CEA).

G. Serum sodium concentration (Na+).

DISCUSSION: The most important test by far is the electrocardiogram, with its capacity to indicate signs of occult heart disease. The second most important evaluation is the hemoglobin concentration, which in this patient may show an anemia related to chronic alimentary tract blood loss that would require correction prior to safe induction of a general anesthetic. Arterial blood gases vary from individual to individual depending primarily on smoking habits and age. Accordingly, each older person should have a resting baseline determination prior to operation. Serum creatinine may show evidence of occult renal disease and is substantially more useful than blood urea nitrogen, which is more vulnerable to transient volume changes. Carcinoembryonic antigen is important to know in many patients with cancer with respect to postoperative followup since in some cases it may be an early herald of recurrent disease. However, it has little to do with the patient's preoperative assessment in terms of risk and preparation for an elective operation. The presence of liver metastases, for example, can be discovered with significant accuracy by palpation at the time of operation, and an elevated carcinoembryonic antigen in no set of circumstances would lead one to withhold colon resection with its relief of potential obstruction and bleeding. Finally, serum sodium concentration in a 65-year-old woman who is admitted electively for resection of the colon is always normal and would be of least value among these tests.