Why Relative Timing Matters
A drug concentration result is interpretable only in relation to when the relevant dose was given and when the sample was collected. A result drawn shortly after a dose and one drawn just before the next dose can differ even when the patient’s regimen and dose amount are the same. The sample’s clock time alone does not tell us which part of the dosing interval it represents.
For therapeutic drug monitoring, record the actual dose event relevant to the sample, the sample collection time, and the dosing interval. These details establish the sample’s position in the interval. That position is essential context for interpreting the result against a monitoring target, but it does not by itself determine what the concentration should be or whether a result is clinically acceptable.
For an oral dose, the relevant event is generally the time the dose was actually taken; for an infusion, the event used as the reference must be stated consistently with the intended interpretation. A prescription time is not proof that a dose was administered at that time. If the actual administration time is known, use it. If it is not known, state that limitation rather than silently substituting the scheduled time.
A dated timestamp is preferable to a time of day alone. For example, “01:00” cannot be compared reliably with a dose recorded at “22:00” unless the date is known. When records use different time zones or daylight-saving conventions, reconcile them before calculating elapsed time. A sample and dose recorded at the same clock time may also require an explicit order: a sample just before a dose is not equivalent to one collected just after it.
Calculating the Position Within an Interval
For a regular regimen, suppose doses are scheduled at \(t_0+k\tau\), where \(k\) is an integer. If the sample occurs after the dose at \(t_0+k\tau\) and before the next dose, then its position is
This formulation simply measures the time from the beginning of the relevant interval and scales it by the interval length. For example, \(r=0.75\) means that three quarters of the interval has elapsed. It does not mean that three quarters of the drug has been eliminated, nor does it specify a concentration. The concentration profile depends on the drug, regimen, and patient, among other factors.
When a dose is taken late, omitted, or otherwise does not follow the regular schedule, do not force the sample into a nominal repeating cycle. Calculate elapsed time from the most recent relevant dose that was actually administered, and record how the observed dosing history differs from the prescribed schedule. The next dose time may also be useful context, especially if the sample was intended to be collected before that dose.
Worked Example: A Sample Collected Early for a Trough
A hypothetical patient takes a dose every 12 hours, at 06:00 and 18:00. A sample intended to be collected immediately before the next 06:00 dose is drawn at 05:00. The previous dose was actually taken at 18:00. Find the elapsed time, the interval position, and the deviation from the intended trough time.
The elapsed time is \(11\) hours: from 18:00 to midnight is 6 hours, followed by 5 more hours to 05:00. The dosing interval is \(\tau=12\) hours, so
The sample was collected 11 hours after the previous dose and one hour before the next scheduled dose. The intended collection was 12 hours after the previous dose, so the timing deviation is \(11-12=-1\) hour: the sample was 1 hour early relative to the intended trough time. It is not exactly a 12-hour postdose sample, even though it is close to the planned collection time.
Early and Late Samples Relative to a Target Time
“Early” and “late” should be defined relative to a specified intended collection time, not used as vague descriptions of a concentration. Let \(t_{\mathrm{target}}\) be the intended time after the relevant dose and \(\Delta t_{\mathrm{target}}\) its intended elapsed time. Define the timing deviation as
If \(e<0\), collection was early by \(|e|\); if \(e>0\), it was late by \(e\); and if \(e=0\), it matched the intended elapsed time. The units of \(e\) are units of time, such as hours. This describes a timing difference, not the size or direction of a concentration change.
Worked Example: A Sample Collected Late Before the Next Dose
In a hypothetical regimen, doses are prescribed every 12 hours, with the next dose due at 10:00. The previous dose was taken at 22:00 the night before. The 10:00 dose has not yet been given, and a sample is collected at 10:30. The intended sample time is immediately before the 10:00 dose, or 12 hours after the previous dose. Calculate the actual elapsed time and timing deviation.
From 22:00 to midnight is 2 hours; from midnight to 10:30 is 10.5 hours. Thus \(\Delta t=12.5\) hours. Relative to the nominal 12-hour interval,
The sample was collected 30 minutes late relative to the intended pre-dose time. Since the next dose had not been given, the last administered dose remains the relevant reference. Here \(r\) exceeds 1 because the sample was collected after the nominal interval had elapsed. That value is a useful warning not to describe the sample as being within the usual 12-hour interval; it does not justify resetting the reference to a dose that had not occurred.
Worked Example: Locating a Sample Between Scheduled Doses
A hypothetical patient takes a dose every 8 hours, with actual doses at 07:15, 15:15, and 23:15. A sample is collected at 18:45 after the 15:15 dose and before the 23:15 dose. Calculate its elapsed time, fractional interval position, and time until the next scheduled dose.
From 15:15 to 18:45 is 3 hours 30 minutes, or 3.5 hours. Therefore,
The sample was collected 3.5 hours after the most recent dose, with 4.5 hours remaining until the next scheduled dose. As a check, \(3.5\ \mathrm{h}+4.5\ \mathrm{h}=8\ \mathrm{h}\), the full interval. The sample is therefore about 43.75% of the way through that interval. This locates it in time; it does not establish a peak, trough, or any particular expected concentration.
Timing Checks and Common Failure Modes
Before interpreting a concentration, check that the dose and collection timestamps are complete and refer to the same time basis. Confirm which dose was actually administered most recently, whether the sample was collected before or after a nearby dose, and whether the stated interval describes the prescribed regimen or the interval that actually occurred. If these details cannot be established, the calculated timing should be reported as uncertain rather than presented as exact.
- Using the scheduled dose instead of the administered dose. A dose taken late changes elapsed time. Use the actual dose time when available, and separately note the scheduled time.
- Ignoring the date. A sample collected after midnight may be only a few hours after a late-evening dose. Use full dates and times to avoid an erroneous negative or excessively long elapsed time.
- Using the wrong dose as the reference. For a sample before the next dose, calculate from the last dose actually taken. Do not calculate from a planned dose that has not yet been given.
- Assuming the same clock time means the same interval position. If dose times shift, a sample at 08:00 can represent different elapsed times on different days.
- Confusing timing with concentration interpretation. A sample that is late for its intended collection is not automatically high, and an early sample is not automatically low. Timing identifies context; interpretation also requires relevant pharmacological and clinical information.
For a regular schedule, a fractional position can provide a compact summary alongside the actual elapsed time. For an irregular history, a simple fraction of the prescribed interval may be misleading. In that case, prioritize the elapsed time since the actual dose and document deviations. Neither representation replaces the original timestamps; retaining them allows the calculation to be checked.
Choosing a Useful Timing Description
Three descriptions are often useful, but they answer slightly different questions. “Hours since the last dose” gives the direct elapsed time. “Fraction of the interval” locates a sample within a regular interval. “Minutes early or late” compares the sample with its intended collection time. Reporting the direct elapsed time together with the interval and any target-time deviation is usually more informative than reporting only a label such as “trough sample.”
| Description | What it tells you | Important limitation |
|---|---|---|
| Elapsed time since actual dose | Time from the most recent relevant administered dose to collection | Requires a reliable dose timestamp |
| Fraction of dosing interval | Relative position in a specified regular interval | Can mislead if dosing was irregular or the sample falls outside the nominal interval |
| Deviation from intended sample time | How early or late collection was relative to a stated target | Requires the intended collection time to be defined |
| Clock time alone | Time of day when the sample was drawn | Does not establish time since dose or interval position |
A common mistake is to reason, “This is a 12-hour regimen, and the sample was drawn in the morning, so it is a trough.” That conclusion is unsupported without the actual dose and sample times and confirmation that the next dose had not already been given. The correction is to calculate the elapsed time from the most recent relevant administered dose, compare it with the intended collection time, and state any uncertainty. A well-documented time position is a foundation for interpretation, not a substitute for it.
Check Your Understanding
Use the stated dose and sample times to answer each question. Solutions are provided after the questions.
- A hypothetical dose is taken at 21:00, and a sample is drawn at 03:00 the next day. What is the elapsed time since the dose?
- A hypothetical regimen has a 10-hour interval. A sample is collected 7.5 hours after the last dose. What fraction of the interval has elapsed?
- A sample intended for 12 hours after a dose is collected 13 hours after that dose, before the next dose is administered. How late is the sample relative to the target?
- Why is “sample collected at 08:00” insufficient to establish whether a sample is a trough?
Solutions. (1) From 21:00 to midnight is 3 hours, then 3 more hours to 03:00, giving \(\Delta t=6\) hours. (2) \(r=7.5\ \mathrm{h}/10\ \mathrm{h}=0.75\), so 75% of the interval has elapsed. (3) \(e=13\ \mathrm{h}-12\ \mathrm{h}=1\ \mathrm{h}\); it is 1 hour late. (4) The clock time does not show when the relevant dose was actually taken or whether a nearby dose had already been administered. Those facts are needed to establish elapsed time and confirm a pre-dose sample.