I did a little research on this last month, and in short, the answer is 'no', seminal fluid will not effect your opk results. I am not sure which opk's you use, but I also did my own little opk experiment to see the difference between a few of them. I tried the First Response, Answer, Walgreens brand, and IC (internet cheapies). As in many things, I think with opk's you definitely get what you pay for. The FR were the most expensive, but most reliable and easiest to read - no major interpretation needed. They also coincided well with my BBT charts (temps and CM). The IC's and Walgreens were the worst, very difficult to read. Answer were not as good as FR either. Once you know your cycle well enough, you shouldn't need to test opk's as often, so for me it's worth spending the extra money to know for sure when LH surge comes-I will continue to use FR. HTH. Best of luck!
