I am sure you can talk to you lawyer and he can tell you better but slandering somes name is a crime and now days you can sue for everything.
Can you sue for defamation of character if someone makes false allegations to CPS? Causing an investigation?
The question is, what would the slandering person gain by CDC believing you are an unfit parent. If it could've resulted to you loosing custody of your children, or you loosing visitation rights, then I believe you have a good case to file for "defamation of character." The negative drug test establishes the report to be false, and the slandering person must either give you and CDC a written formal apology, or punish the slandering in civil court, by seeking punitive damages for their irresponsible statements. Write him/her a letter of your intent to litigate (sent via certified mail with return receipt, or email her if you have proof the email address in FACT belongs to her.), and stipulate how much $$ you've lost in taking time off from work to resolve this with the CDC.
If he/she fails to issue a formal written apology, along with monetary compensation for loss of wages, then you can file in small claims court. Say you claim $100 in loss wages for having to take time off to come into CDC's office. File for $100 actual damage in loss wages, and maybe $1,900 in punitive damages, for a total of $2,000 in damages on your complaint. Punitive damages is exemplary damages; you're asking the court to punish the defendant for his/her behavior. The court may award you none of it, some of it (they agree with your argument, but feel the punitive damage is too high), or all of it.
If he/she fails to issue a formal written apology, along with monetary compensation for loss of wages, then you can file in small claims court. Say you claim $100 in loss wages for having to take time off to come into CDC's office. File for $100 actual damage in loss wages, and maybe $1,900 in punitive damages, for a total of $2,000 in damages on your complaint. Punitive damages is exemplary damages; you're asking the court to punish the defendant for his/her behavior. The court may award you none of it, some of it (they agree with your argument, but feel the punitive damage is too high), or all of it.
Anyone can sue for almost anything. Can you afford to hire a lawyer for the suit? Would it be worth it? Balance your costs and the hassle against what you might gain and the other person might lose. Can you *prove* malicious intent on the part of the person who defamed you? Can you *prove* financial loss as a result of the inaccurate report to CPS?
Will it be worth it. That is the question. You must be able to prove damages, in most states, and if you win, will the damages make you whole, or with lawyer and court costs, will you be wasting time and money. 
I know who did this and it has made it so NONE of my grandkids are allowed at my home now. But what she told was all not true. It is proving to be so. ( which we already knew) We were reported as having let a 10 yr old watch porn and that I threatened to cut a child's throat. Ive had to submit to drug tests. I DO NOT DO DRUGS! The case is not closed as of yet, but when it does I am suing the pants off this woman! I do not want monetary damages, but I want to show her that you just cannot go around telling lies on people and have no consequences.