How long does one use of cocaine stay in your system?

I smoked crack five days ago. It was the first time since about one year ago that I did some. And I have not smoked anymore since five days ago. I gave myself an immunassay home drug test today with cutoff levels for the cocaine metabolite "benzoylecgonine" and a few others at 150 ng/ml. My test was negative. because I only did a small amount and had not done any before or after, five days was enough to rid my body of the metabolites.