Is 18 days enough time to get a quantitative blood test done for pregnancy?

If your periods are regular (24-32 days apart) and the "18 days" represents time after your normal period was due, it is enough time to obtain a quantitative HCG level.

Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully.