11.2 Tolerance Tests & TDM Scheduling

Quick Answer: For glucose tolerance tests, draw the fasting baseline, administer the ordered glucose load, then collect each timed specimen on the exact interval schedule while the patient stays fasting and at rest. For therapeutic drug monitoring, draw troughs just before the next dose and peaks at the drug-specific post-dose time—and record actual times so clinicians can interpret the level.

Domain III-C and III-D address tolerance testing and therapeutic drug monitoring (TDM) scheduling. These are the classic "clock-driven" collections where a technically perfect venipuncture still produces a useless result if the time is wrong. RPT candidates must know the sequencing, the documentation burden, and the coordination with nursing and pharmacy.

Oral Glucose Tolerance Testing (OGTT): Purpose and Preparation

An oral glucose tolerance test measures how the body handles a standardized glucose load. It is used in evaluating diabetes and impaired glucose tolerance, and related protocols appear in gestational diabetes testing. Exact dose and interval schedules are ordered by the provider and defined by laboratory procedure—always follow the order and procedure manual for that patient.

Typical preparation themes (confirm locally):

• Overnight fast (commonly at least 8 hours; often up to 16 hours maximum in classic protocols)—water is usually allowed; caloric intake is not.
• Usual carbohydrate intake in the days before testing when the protocol requires it.
• Morning testing preferred; the patient should remain seated/resting during the test when possible.
• Hold confounding factors per protocol (for example, smoking during the test is often restricted).

If the patient did not meet the fasting requirement, do not start the challenge until the provider or procedure allows an alternative plan. Starting an OGTT on a nonfasting patient wastes the appointment and produces misleading curves.

OGTT Collection Sequence

A standard adult diagnostic pattern uses a 75 g oral glucose load (pediatric dosing is weight-based with a maximum, commonly 1.75 g/kg up to 75 g). Many protocols sample as follows:

The standard nonpregnant 75-g, 2-hour OGTT (per ADA) draws at 0 hours (fasting baseline) and 2 hours after ingestion. Variations exist: a 1-hour 50-g screen (gestational, often nonfasting) and a separate diagnostic 100-g, 3-hour test (fasting, 1, 2, and 3 hours) with different cutoffs and intervals are not the same workflow. Read the order: "1-hour glucola" is not the same as a "3-hour GTT."

Operational Rules That Protect Validity

1. Draw baseline first, label it as fasting/0-hour.
2. Administer the glucose beverage and note the finish time—this is your zero for subsequent intervals in most phlebotomy workflows.
3. Set timers for each remaining draw; do not rely on memory during a busy morning.
4. Keep the patient fasting except for the glucose dose until the test ends (no juice "because they feel shaky" unless a protocol-stopping adverse event occurs—then follow emergency/nursing guidance and document).
5. Collect each interval into the correct tube (often gray-top fluoride for glucose stability—follow lab procedure) and label with the time point as well as patient identifiers.
6. If a draw is late or early, document the actual time; large deviations may require provider notification or test invalidation per policy.

Adverse symptoms (vomiting the load, syncope, inability to continue) can abort the test. Notify the provider/nurse, document what was retained of the dose and which samples exist, and do not invent missing time points.

Therapeutic Drug Monitoring: Why Timing Is the Result

TDM measures drug concentration to guide dosing—efficacy, toxicity, compliance, and changes in organ function. For many drugs, the number on the report is meaningless without knowing whether it is a trough, peak, or random level and when the last dose was given.

Representative Timing Patterns

Exact minutes vary by drug monograph and hospital protocol. Know these common patterns conceptually:

You are not expected to memorize every pharmacokinetic table for the RPT, but you are expected to: (1) recognize peak vs trough on the order, (2) coordinate with nursing so doses are not given before a trough is drawn, (3) schedule peaks from the end of infusion when that is the rule, and (4) document times.

Coordinating With Nursing and Pharmacy

TDM failures are usually communication failures:

• Arrive for a trough to find the nurse already hung the antibiotic—now the level is not a trough. Do not relabel it as trough. Draw only if the provider wants a random level, and document the relationship to the dose.
• For peaks, confirm infusion start/end times before you start the clock.
• If you will be late for a trough, ask nursing to hold the dose until collection when clinically appropriate and ordered.
• Never draw TDM specimens from a line lumen used to infuse that same drug without following line-draw policy—contamination falsely elevates levels. Prefer a peripheral stick when required by procedure.

Documenting Draw Times

Documentation is part of the specimen:

• Record date and exact clock time of collection on the label/requisition/LIS prompt.
• Record or verify last dose time, dose amount, and whether the level is peak, trough, or random when the system requires it.
• If the ordered time was 09:00 and you collected at 09:12, write 09:12, not 09:00. Honest times let pharmacists interpret; fictional punctuality endangers dosing decisions.
• For OGTT, label each tube with the interval (0, 30, 60, 120, etc.) and actual time.

Guidance often cited for peaks you cannot hit exactly: collecting slightly late with accurate documentation is preferable to collecting early (still in distribution for some drugs). Always follow your laboratory's TDM collection guideline.

Putting III-C and III-D Into One Shift Story

Imagine 08:00: Patient A starts a 2-hour OGTT (baseline drawn, glucola finished 08:05 → 2-hour draw due 10:05). Patient B has a gentamicin trough due 08:25 before an 08:30 dose. Patient C has routine labs. Sequence: protect B's trough (and dose hold if needed), keep A's timers running, fit C around those anchors, return for A's 10:05 draw without letting anyone feed Patient A early. That is time management as clinical practice—not as abstract productivity.

Exam Traps for III-C and III-D

• Starting the OGTT clock when the drink is handed over instead of when ingestion is completed (follow your procedure—most timing keys off completion).
• Allowing snacks during OGTT.
• Drawing a trough after the dose and still calling it a trough.
• Drawing digoxin shortly after a dose and treating it as interpretable steady concentration.
• Omitting actual collection time on TDM or tolerance specimens.
• Collecting a peak from the infusion line without proper protocol, contaminating the sample.

Master the clock, the labels, and the handoffs. Tolerance tests and TDM are where organization becomes patient safety.