Timed Collections and the PK Window

Part of Research Nurse (RN) Training: Specimens and Chain of Custody.

About 3 minutes. Free, in your language, no account needed.

Why this matters

Pharmacokinetic sampling asks what the drug concentration is at specific moments after dosing. The moment is the measurement.

A PK series taken at the wrong times does not produce a slightly wrong curve. It produces a curve of something else.

The anchor is the dose

Every time in a PK schedule is measured from the dose, not from the clock, not from the start of the visit.

That means the dose time must be exact. If the dose time is recorded as 09:00 when it was actually 09:14, every sample in the series is mis-anchored by fourteen minutes.

Write the dose time as it happens, to the minute.

Windows get tighter near the peak

Early samples usually have narrow windows. Later ones are wider.

That is because concentration changes fastest early. A five-minute error at fifteen minutes post-dose matters far more than a five-minute error at eight hours.

Know which of today's samples are the tight ones and protect those first.

When you cannot hit the window

Take the sample anyway, at the time you can, and record the actual time.

A sample outside its window, correctly timed in the record, can often still be used with a note. A missing sample cannot be used at all. A sample recorded at a time it was not taken is worse than either.

Also record why, briefly. Difficult access. Participant in the bathroom. Line failed.

Practical preparation

Set out the tubes in order before the participant arrives. Write the schedule where you can see it. Use a timer rather than watching a clock.

For an intensive day, agree in advance who draws, who labels and who records. A PK day with one nurse doing everything is where windows get missed.

In practice

A participant needs to leave before the eight-hour sample. They are adamant.

You do not pretend it was taken. You take what you can, record the missed timepoint as a deviation, and tell the coordinator so the sponsor knows the series is incomplete.

An honest gap is a known limitation. An invented value is fraud.

What this looks like when it goes wrong

A nurse records all PK samples at their scheduled times because the visit ran smoothly and the actual times were close.

Analysis shows an unusual concentration curve for that participant. The pharmacologist cannot tell whether it is biology or timing, because the recorded times are too perfect to be real.

The whole profile is excluded.

Key takeaways