What a clock letter is
When a laboratory reports a non-negative result, the Medical Review Officer must speak with the employee before anything is final. The interview is the employee's chance to explain, a prescription or a medical situation, and the rule requires the MRO to make and document attempts to reach them, spaced over a 24-hour period, before turning to you (49 CFR 40.131). When those attempts fail, the MRO formally notifies the employer's designated representative, and a clock starts. Under 49 CFR 40.133 the MRO may verify the result without the interview once more than 72 hours have passed since you documented your contact with the employee and told them to call, or, if neither of you can reach the employee at all, ten days after the MRO received the laboratory result. That notice is the "clock letter": a deadline for the employee to speak with the MRO, while the result is still unverified.
Your three steps
Privately tell the employee to contact the MRO. Give them the callback number from the notice and their specimen ID, and tell them to make the call now. Privately means privately: this conversation is between you and the employee, nobody else.
Write down the date and time you told them. Your notification is itself a regulated step. The 72-hour window runs from the contact you document, so the timestamp matters.
Inform the MRO of the date and time you made contact. The rule has you tell the MRO directly; if your employer has elected to route this notice through a service agent, make sure it reaches the MRO the same day. Then let the process run.
Pending is not positive
Until the MRO verifies the result, there is no result. There is a laboratory finding awaiting medical review, and that review exists precisely because legitimate explanations are common. So no employment action yet: no announcement, no suspension, no termination. For DOT-covered roles the rule goes one step further. 49 CFR 40.21 prohibits standing an employee down, that is, removing them from safety-sensitive duty because of a laboratory report the MRO has not yet verified, unless your DOT agency has granted you a written stand-down waiver under that section. A company policy cannot substitute for that waiver. You may remove the employee only on grounds independent of the pending result: a reasonable-suspicion determination by a trained supervisor under 49 CFR 382.307, or the 24-hour alcohol rule in 382.307(e)(2). Acting early on the laboratory report itself creates a larger problem than the one you were worried about.
What the employer receives
The medical conversation belongs to the MRO and the employee. What the employer receives is the verified result: negative, negative-dilute, positive, refusal to test, or cancelled, and for a verified positive, the drug or metabolite it was positive for (49 CFR 40.163). Quantitative values and the employee's medical explanation stay with the MRO. Some results carry an immediate duty of their own: an invalid result with no medical explanation, a positive that had to be cancelled because the split specimen could not be tested, and a negative-dilute in the creatinine range the MRO flags each require an immediate recollection under direct observation, with no advance notice (49 CFR 40.67(a)). Read the MRO's report for that instruction.
After verification: two rights the employee keeps
If the MRO verifies a positive or a refusal for adulteration or substitution, the employee has 72 hours from the MRO's notification to ask for a test of the split specimen, and the employer must make sure that test happens without waiting for the employee to pay for it (49 CFR 40.171, 40.173). And if the MRO verified the result without an interview, the employee has 60 days to show that illness, injury, or other circumstances unavoidably prevented contact, which can reopen the verification (49 CFR 40.133(d)).
Our DER support line handles clock letters routinely. If one just landed on your desk and you want a person to walk through it with you, call 732-869-2900, answered Monday to Friday, 8:00am to 4:00pm Eastern, or schedule a call.