5.2 Following Manufacturer Instructions for Waived Tests
Key Takeaways
- AMT I.2.B.2 names four IFU traps: wrong population versus intended use or limitations, wrong read time, wrong storage, and wrong specimen type (whole blood versus serum; throat versus nasopharyngeal).
- Reading a lateral-flow device too early can miss a true line; reading too late can produce evaporation or ghost lines—both are invalid, not "almost positive."
- The printed kit expiration applies to unopened product stored as labeled; open-vial or in-use dating after opening is often shorter and is the earlier discard date.
- Store reagents at the labeled range (refrigerator versus room temperature); do not freeze unless the IFU says freeze, and equilibrate refrigerated kits before use when the insert requires it.
- An unapproved specimen type is both an invalid result and off-label use that defaults the method to high complexity.
AMT I.2.B.2 tells you to follow the manufacturer's instructions for use (IFU) or package insert precisely so you avoid four named failures: testing a population the intended use does not cover, reading outside the labeled time frame, storing reagents incorrectly, and using the wrong specimen type. Each failure can produce a wrong patient result. Each is also off-label use that defaults the method to high complexity (Section 5.1). For a CMLA, the insert is not background reading. It is the procedure.
Quick Answer: Match the patient to intended use and limitations, the specimen to the insert (whole blood versus serum; throat versus nasopharyngeal), the cassette to the read window, and the kit to labeled storage and dating. Too early, too late, frozen-when-forbidden, or open-vial past in-use dating is an invalid test—not a result you "interpret with judgment."
Intended use and limitations: the population trap
Every waived kit has an intended use (who and what it is for) and limitations (who and what it is not for). Performing the test on a population outside that language is AMT's first named trap.
| Kit family | Typical intended use (examples—always read this insert) | Off-label population / limitation |
|---|---|---|
| Rapid influenza immunoassay | Symptomatic patients within the labeled days of onset | Asymptomatic "clearance" screening when the IFU is diagnostic only; testing on day 8 when the insert says within 3–4 days of symptoms |
| Group A strep antigen | Symptomatic pharyngitis; throat swab | Screening well contacts; using saliva or a cheek swab |
| Urine hCG | Detecting pregnancy in urine from women of childbearing age | Using the cassette as a tumor-marker test; testing a male urine without an IFU that allows it |
| Waived glucose meter | Capillary whole blood as labeled; some meters exclude neonates or critically ill patients | Neonatal or intensive-care arterial samples on a meter whose limitations forbid them |
| Fecal occult blood | The labeled screening or diagnostic use | Using a screening-only card to "rule out" an upper gastrointestinal bleed the insert does not address |
Scenario. A parent wants a rapid influenza test on an asymptomatic child before a birthday party. The IFU says the device is for patients with influenza-like illness within four days of onset. Running it anyway is a population/intended-use failure. Explain the limitation, offer collection only if a clinician orders a method indicated for that use, and do not treat a negative off-label cassette as proof the child is not infectious.
Limitations also hide in small print: "not for use with patients receiving hCG-containing medications," "not evaluated in children under two," "not for quantitative monitoring." If the stem gives an excluded population, the correct move is do not run this kit, not "the control line will save us."
Read the test in the labeled time frame
Lateral-flow and many immunoassay cassettes specify a read window: a minimum time so the conjugate can migrate, and a maximum after which the result is invalid. AMT's second trap is not reading the test in an appropriate time frame.
| Timing error | What you may see | What to do |
|---|---|---|
| Too early (for example, peeking at 2 minutes when the IFU says read at 10) | Blank window or a faint incomplete line | Wait until the minimum time; do not report the peek |
| Inside the window | Control line present; patient line present or absent per IFU | Report per the insert |
| Too late (for example, 25 minutes when the IFU says do not read after 15) | Evaporation lines, ghost lines, or dried-out windows | Invalid — do not report; repeat with a new device if the specimen is still acceptable |
Too early undercalls true positives because the antibody-antigen-conjugate line has not formed. Too late overcalls because liquid evaporates and leaves a smear that looks like a line. A faint line after the stop time is not "weakly positive." It is a result you are not allowed to interpret.
Set a timer. Do not stack ten cassettes and "read them when you get back from lunch." If you find an unread device past the maximum time, it is invalid even if a line is pretty. If the IFU says read at exactly 15 minutes, 15 minutes is not 20.
Some instruments beep and lock the result at the validated time. Do not override a lockout, photograph a late window, or transcribe a late visual read "because the patient is waiting." That is the same timing modification as extra incubation in Section 5.1.
Scenario. A waived strep cassette IFU says read at 5 minutes and do not read after 10. At 6 minutes the window is negative with a valid control line; you report negative (and still follow confirmatory rules in Section 5.3). At 18 minutes a ghost line appears. You do not call the provider to "upgrade" the result. The late line is outside the time frame.
Store reagents and supplies correctly
AMT's third trap is not storing reagents or supplies correctly. Storage is a labeled range, not "the drawer that is convenient."
- Refrigerator versus room temperature. Many unopened cassettes are 2–30 °C; many liquid controls are 2–8 °C. Putting controls on a warm windowsill, or leaving a "room-temp" kit on a loading dock in July, is a storage failure.
- Do not freeze unless the IFU says freeze. Accidental freezing can crack cassettes, separate conjugates, and destroy controls. If a kit that says "do not freeze" froze in transit, quarantine and discard per policy—do not thaw and use it.
- Equilibration. If the insert says bring refrigerated devices to room temperature for 15–30 minutes before use, a cassette run ice-cold can fail to flow or can time incorrectly. Do not warm kits on a heat block, in a microwave, or against a body to "speed it up" unless the IFU says so.
- Humidity, light, and pouch integrity. Open the foil pouch only when you are ready to test. A damaged pouch, a desiccant that has turned, or cassettes left out overnight are not saved by a control line.
- Do not use expired product. The printed expiration is for unopened kits stored as labeled. "Just this once" is still expired, still off-label, still a high-complexity default.
Open-vial dating versus kit expiration
Two clocks run on liquid reagents and opened control bottles:
| Clock | What it is | Which date wins |
|---|---|---|
| Printed kit / vial expiration | Stability of the unopened product stored as labeled | Discard when this date arrives, even if you opened the bottle yesterday |
| Open-vial / in-use dating | Stability after opening (for example, 30 or 90 days, or "until printed expiration, whichever is first") | Discard at the earlier of in-use dating and the printed date |
Write the open date, your initials, and the new discard date on the vial when you open it. A control bottle that expires 2027-03-01 but was opened 90 days ago with 30-day in-use dating is expired now. Using it is both a QC failure and an IFU storage/dating failure. Chapter 17 returns to inventory expiration versus open date as a clerical competency; here it is how you keep the waived method valid.
Scenario. Influenza kits belong at 2–30 °C. A box sat in a parked car at 40 °C. You do not "cool it back down" and test tonight's clinic. Stability data were not collected for that excursion. Quarantine the lot, notify the supervisor, and use in-range inventory.
Correct specimen type: whole blood versus serum; throat versus NP
AMT's fourth trap is employing an incorrect specimen type, with the official examples whole blood versus serum and throat versus NP.
| Ordered waived test | IFU specimen (typical) | Wrong specimen that looks "close enough" |
|---|---|---|
| Fingerstick glucose, some waived chemistry meters | Capillary or venous whole blood as named | Serum or plasma poured from a centrifuged tube onto a whole-blood strip |
| Some waived lipid or hCG blood cassettes | Serum or plasma only | Fingerstick whole blood |
| Rapid Group A strep antigen | Throat swab from tonsils/posterior pharynx | NP swab leftover from a flu workup; saliva; buccal swab |
| Rapid influenza / RSV antigen | Often nasopharyngeal swab or aspirate | Throat swab when the insert lists NP only |
Whole blood contains cells; serum does not. Hematocrit, clotting, and matrix effects change how a strip or meter reads. A serum drop on a whole-blood glucose strip is not a creative capillary. An NP swab is not a throat swab: different flora, different antigen load, different clearance. Using the leftover NP swab on a throat-only strep kit (or a throat swab on an NP-only influenza kit) is the exact AMT example of incorrect specimen type—and it is the Section 5.1 modification that defaults the test to high complexity.
Collect the specimen the insert names, in the labeled transport medium if one is named, at the labeled volume. Do not dip a cassette into a transport tube the IFU forbids. Do not "rinse" a dry swab with tap water. If the collector already used the wrong site, do not test that swab on this kit. Recollect or send a method whose IFU matches the specimen you have.
Putting the four traps together
A busy shift will offer all four at once: an asymptomatic employee wants a flu test (population), the only kit on the shelf expired yesterday (storage/dating), the collector sent a throat swab for an NP-only device (specimen), and nobody can read the cassette until after lunch (timing). The professional sequence is refuse the off-label population, discard or quarantine the expired lot, recollect the correct swab, and read inside the window. Any one of those shortcuts is an IFU failure. All four together is still one rule: follow the insert or do not run the test.
Exam traps
- Calling a late evaporation line a weak positive.
- Treating serum and whole blood as interchangeable because "it is still blood."
- Treating throat and NP as interchangeable because "it is still respiratory."
- Freezing leftover cassettes to stretch dating.
- Using an opened control until the printed box date and ignoring in-use dating.
- Testing an excluded age group or asymptomatic patient because a provider "just wants a peek."
If the insert does not describe the patient, the specimen, the clock, or the storage condition in front of you, you do not have a waived test. You have a stop.
A waived influenza immunoassay lists nasopharyngeal swab as the only approved specimen. The collector submits a throat swab. What should the CMLA do?
A lateral-flow cassette IFU says to read the result at 10 minutes and not after 15 minutes. The cassette is read at 25 minutes and a faint line is present. How should that reading be handled?
Which storage and dating practice follows manufacturer instructions for a waived kit?