Would a leukocyte value of 12.5x10(9)/l be a reason to look for leukemia?

Usually not necessary, especially if you have symptoms of an infection. The large majority of persons in this situation do not have leukemia. However, if no obvious infection, the WBC (leukocyte) count should be repeated, including differential count to determine which kind of WBC is elevated. Depending on those results, conceivably other tests to evaluate possible leukemia would be recommended.

All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, an infection is a more likely explanation for your results. Wish you good health!