Situation: A 45-year-old man is on day 5 after a living-donor kidney transplant. He takes tacrolimus at 8:00 AM and 8:00 PM, mycophenolate, and prednisone. He has an indwelling urinary catheter, and his urine output over the past day was 150–200 mL/h. A tacrolimus trough level is ordered. When should the blood be drawn?

A trough level measures the lowest drug concentration, which occurs just before the next dose. The morning dose is held until the sample is drawn and then given on time. Levels drawn at other times cannot be compared with the trough target range.

A trough level is defined as the lowest serum drug concentration during a dosing interval, and it is measured immediately before the next scheduled dose is due. For a patient taking tacrolimus at 8:00 AM and 8:00 PM, the blood sample must be drawn just before the 8:00 AM dose, with the morning dose held until the sample is collected. This timing ensures the result reflects the true nadir concentration and can be compared against the established trough target range.

Therapeutic drug monitoring (TDM) for tacrolimus relies on consistent sampling relative to dosing because tacrolimus has a narrow therapeutic index. A level drawn two hours after the dose captures the absorption or distribution phase, not the trough. A midday sample falls between peak and trough and cannot be interpreted using trough reference values. A bedtime sample after the evening dose is also inappropriate because it does not represent the pre-dose nadir for the morning interval.

The clinical consequence of incorrect sampling timing is significant. If a blood sample is drawn too early after a dose, the measured concentration is falsely elevated. This can lead a clinician to reduce the tacrolimus dose unnecessarily, placing the transplant recipient at risk of acute rejection. Conversely, a sample drawn late—after a missed or delayed dose—may be falsely low and prompt an unsafe dose increase, increasing the risk of nephrotoxicity, neurotoxicity, and other adverse effects.

The morning dose is held until the trough sample is drawn, and then the dose is given on time to maintain the regular 12-hour schedule. This practice prevents the morning dose from contaminating the trough measurement while preserving the patient’s dosing rhythm. In the inpatient setting, coordination between nursing staff and the phlebotomy team is essential so the draw occurs before the 8:00 AM administration.

The importance of sampling timing is not unique to tacrolimus. The same principle applies to other drugs monitored by trough levels, including cyclosporine, vancomycin, and voriconazole. Key point! A case report on voriconazole TDM demonstrated that misinterpretation of a drug concentration occurred because the sample was not collected at the correct time relative to dosing ^[1]. The clinical pharmacist identified that the timing error—not true drug accumulation—explained the unexpectedly high result. This reinforces that the value of any drug level depends entirely on when the sample was drawn relative to the last dose.

For this patient on day 5 after living-donor kidney transplant, the high urine output of 150–200 mL/h indicates good early graft function, but tacrolimus dosing still requires careful TDM to balance immunosuppression against toxicity. Watch out! Drawing the level at noon or after the evening dose produces a value that cannot be matched to the trough target range, and any dose adjustment based on such a value would be clinically unsafe.

Sampling time	Type of level	Interpretation for tacrolimus
Just before 8:00 AM dose	Trough	Correct; reflects lowest concentration and is comparable to target range
Two hours after 8:00 AM dose	Absorption/distribution phase	Falsely elevated; not a trough
Noon (midway between doses)	Random level	Cannot be interpreted using trough reference values
After 8:00 PM dose	Post-dose level	Not the pre-dose nadir for the next interval

In nursing practice, the correct sequence is to verify the order for a trough level, ensure the next dose has not yet been given, draw the blood sample, document the exact collection time, and then administer the scheduled dose. Documenting the precise time of both the last dose and the blood draw is essential for accurate interpretation by the clinical pharmacist or provider. A trough level without a documented dosing time has limited clinical utility because the interval between dose and sample determines whether the result is valid ^[1].