5.3 Confirmatory and Supplemental Testing

AMT I.2.C.1 is a single instruction with three named applications: follow the manufacturer's instructions regarding the necessity for confirmatory and supplemental testing. A waived screen can be clinically useful and still be presumptive. The package insert—not hallway custom—tells you when a second method is required before anyone treats the result as final. The CMLA may perform the waived screen. The confirmatory methods AMT lists are typically moderate- or high-complexity work. Learn when confirmation is required and how to handle and send the specimen. Do not learn to operate a gas chromatography/mass spectrometry (GC/MS) instrument.

Quick Answer: Read the IFU's confirmatory section. Urine drug immunoassay positives go for GC/MS or liquid chromatography/mass spectrometry (LC/MS). Negative rapid strep screens often need culture or an approved confirmatory molecular test; influenza immunoassays follow their own IFU confirmatory language. Cultures may need direct fluorescent antibody (DFA) or approved molecular confirmation. Collect the extra swab or aliquot, label it, and send it. Do not report a screen as confirmed just because you repeated the same waived kit.

Screen versus confirmatory versus supplemental

A screening test is designed to be fast and, often, highly sensitive or convenient. It may cross-react, miss low antigen loads, or detect a drug class rather than a specific compound. A confirmatory test uses a more specific method on the same (or paired) specimen to verify the screening result. Supplemental testing is the related idea used in serology algorithms: a second, different method that resolves a reactive screen (classic example: a waived human immunodeficiency virus (HIV) screen that the IFU says must be followed by laboratory supplemental/confirmatory testing—historically Western blot, now an HIV-1/HIV-2 antibody differentiation immunoassay and nucleic acid testing per the current Centers for Disease Control and Prevention (CDC) algorithm).

Repeating the same waived cassette is not confirmation. Running a second brand of immunoassay on the same urine is usually still a screen. Confirmation means the method the IFU (and facility procedure) names.

Urine drug screening: GC/MS or LC/MS

Waived urine drug tests are immunoassays. They detect drug classes at a labeled cutoff (for example, cannabinoids, cocaine metabolites, opiates, amphetamines, phencyclidine). A line or instrument flag means the specimen screened at or above the cutoff, not that a medical review officer has identified a named illegal substance. Poppy seeds, some prescriptions, and cross-reacting compounds can produce presumptive positives. Dilute urine can produce false negatives.

AMT's confirmatory methods for urine drug screening are GC/MS or LC/MS. Those separations plus mass-spectral identification are high complexity. A CMLA does not inject the extract, interpret ion ratios, or "confirm" by repeating the cup.

When it is required. Follow the IFU and the order: workplace, legal, pain-management, and many clinical protocols treat immunoassay positives as presumptive until GC/MS or LC/MS is reported. Do not release "cocaine confirmed" from a waived cup. If the insert says positives must be confirmed, the screen is incomplete without the send-out.

How the CMLA handles the specimen.

• Collect the volume the IFU and the reference laboratory both need so confirmation is possible; do not empty the cup into a cassette and discard the rest.
• Check temperature, appearance, and any adulteration or validity strips the protocol requires (legal collections also use chain of custody—Chapter 8).
• Label the pour-off or the original container with two identifiers; cap it; bag it.
• Store and ship at the temperature the confirmation laboratory specifies (often refrigerated; some drugs require frozen aliquots—follow that IFU, not the waived cup's room-temp note).
• Document the presumptive result as unconfirmed / pending confirmation per SOP. Do not consume the only aliquot on extra waived retests.

Scenario. An occupational cup flags opiates. You record a presumptive positive, secure the remaining urine, complete the custody form if this is a legal collection, and send it for GC/MS or LC/MS. You do not tell the supervisor "the patient is positive for heroin" from the cup. Immunoassays do not distinguish morphine, codeine, and many synthetics the way mass spectrometry does.

Rapid strep screen and influenza immunoassay

Rapid antigen detection tests (RADTs) for Group A Streptococcus are common waived kits. They are typically specific when positive in a symptomatic patient, but sensitivity is imperfect, especially in children. Many IFUs still say: confirm negative rapid strep results with culture or an approved confirmatory molecular test. That pairing is AMT I.2.C.1.b. Adult practice may not back up every negative (rheumatic fever risk is lower), but the exam answer is follow the manufacturer instructions and the facility SOP, not a memory of a guideline year.

How the CMLA handles the specimen.

• Use a throat swab from the tonsils and posterior pharynx (Section 5.2). NP, saliva, and buccal swabs are the wrong type for a throat-only kit.
• Dual-swab when policy requires confirmation: one swab for the waived antigen test, one placed into transport medium for culture or molecular confirmation before you leave the patient. Do not try to "wash" a used antigen swab into a culture tube unless the IFU says that swab is validated for both.
• If the rapid result is negative and confirmation is required, send the second swab promptly. Do not hold it unrefrigerated on the bench until the next courier "because it is probably negative anyway."
• If the rapid result is positive, many IFUs do not require culture confirmation because specificity is high—but still follow this insert. Never skip a required backup because the clinic is busy.
• Culture of Streptococcus pyogenes and high-complexity molecular identification are not independent CMLA testing. Some FDA-waived molecular strep devices exist; those remain waived only if you follow that device's IFU. Using a waived molecular cartridge as the "approved confirmatory molecular test" is correct when the SOP names that cartridge. It is not permission to plate blood agar and interpret hemolysis as a CMLA-run culture.

Influenza immunoassay sits in the same AMT bullet because the workflow is the same idea: rapid antigen is a screen with limited sensitivity (and, when community prevalence is low, limited positive predictive value). CDC and many IFUs tell laboratories to confirm selected results with a more specific molecular method. Collect the NP swab or aspirate the influenza insert names (not a leftover throat swab). If confirmation is required, keep enough specimen, or collect a paired swab, in the transport the molecular laboratory validates. Do not freeze a swab the molecular IFU says never to freeze, and do not leave it at room temperature past the labeled hold time.

Scenario. A seven-year-old with fever and exudative pharyngitis has a negative waived strep antigen. The IFU says negatives must be confirmed by culture or molecular testing. You already collected a second throat swab in transport medium. You report the rapid result as negative pending confirmation, send the culture or waived-molecular confirmatory per SOP, and you do not tell the family "it is definitely not strep" from the cassette alone.

Confirmatory testing for cultures: DFA or approved molecular tests

AMT I.2.C.1.c: confirmatory testing for cultures uses DFA or an approved confirmatory molecular test. Direct fluorescent antibody testing uses a fluorescent-tagged antibody to detect antigen on a smear from a specimen or a culture isolate, read with a fluorescence microscope. Classic teaching examples include viral DFA panels (respiratory syncytial virus, influenza, parainfluenza, adenovirus) and organism-specific DFAs. Molecular confirmation (nucleic acid amplification) is increasingly the method laboratories use instead of or after culture.

These methods are not waived cassette reads. Fluorescence microscopy interpretation and most culture identification are moderate or high complexity. A CMLA may, within assistant scope and SOP, label the isolate or smear, prepare a send-out, or assemble reagents a scientist will read—and must not independently issue a DFA interpretation or a molecular confirmatory result unless the laboratory's CLIA personnel rules and the method's complexity actually authorize that person and that certificate. On this exam, assume they do not.

How the CMLA handles the specimen.

• Make sure the culture or primary specimen is identified, dated, and stored at the temperature the confirmatory method requires.
• Do not discard plates or leftover transport until the SOP says the workup is complete; confirmation often uses the same isolate.
• Package slides or tubes per the reference laboratory: leakproof, biohazard-labeled, with the test requested (DFA versus molecular) written clearly so the wrong confirmatory is not performed.
• Do not treat a preliminary culture colony morphology as a final identification when the IFU or SOP requires DFA or molecular confirmation.

Reporting while confirmation is pending

Release what the IFU allows you to release, and label it honestly:

• "Presumptive positive cocaine metabolite, confirmation pending" is a complete assistant action.
• "Positive for cocaine" from a waived cup is not.
• "Rapid Group A strep antigen negative; culture sent" is complete when backup is required.
• "Strep negative—no further testing" is wrong when the insert still requires confirmation.

Critical-value and provider-notification rules (Chapter 17) still apply to the screen when policy says a presumptive result must be called. Calling a presumptive does not cancel the send-out.

Exam traps

• Repeating the same immunoassay and calling the second cup "GC/MS confirmation."
• Reporting a child's negative rapid strep as final when the IFU requires culture or molecular backup.
• Using a throat swab to "confirm" influenza because a strep swab was handy.
• Claiming CMLA certification authorizes independent GC/MS, DFA, or culture workups.
• Discarding leftover urine or the second swab so confirmation is impossible.

The CMLA who can run the waived screen, stop at the insert's confirmatory sentence, and send the right specimen has completed I.2.C. Operating the confirmatory analyzer is someone else's complexity.