MT MLO common mistakes
MT MLO · Understand the traps and how to avoid them.
Rejecting a sputum specimen solely because it is thin or watery without first performing a Gram stain to assess squamous epithelial cell and neutrophil content.
Candidates memorize 'saliva is unacceptable' and apply a visual shortcut instead of the objective Murray-Washington-style screening criteria (for example, fewer than 10 squamous epithelial cells and more than 25 neutrophils per low-power field). Poor-quality specimens are common, so the shortcut feels efficient.
What to do instead
Before deciding, state the screening criteria explicitly and apply them to a described Gram stain result. Practice classifying borderline specimens: a watery specimen with few epithelial cells and abundant neutrophils is still acceptable; a thick mucoid specimen loaded with epithelial cells is not.
Accepting a urine culture for a catheterized or clean-catch specimen without checking transport time and preservative status, then interpreting a mixed culture as contamination rather than as a pre-analytical delay artifact.
Candidates focus on the culture result and skip the collection history. Delayed transport allows overgrowth of enteric flora, producing a falsely 'mixed' or falsely high colony count that mimics contamination or true infection.
What to do instead
Build a habit of writing the collection-to-receipt interval and preservative (boric acid tube versus plain) before reading the plate. If the interval exceeds the validated window, flag the result as limited rather than calling contamination, and recommend recollection.
Reporting a blood culture isolate's susceptibility from a direct colony suspension without performing a purity check or confirming the identification from isolated colonies.
Time pressure and the desire to report quickly lead candidates to skip the subculture purity plate. Mixed or misidentified isolates then generate susceptibility results that belong to a different organism.
What to do instead
Always verify a single colony morphology on the purity plate and confirm identification (for example, MALDI-TOF or biochemical panel) from an isolated colony before setting up susceptibility testing. Treat discrepant morphology as a stop signal.
Applying CLSI interpretive breakpoints without confirming the correct table version, organism group, and testing method, and ignoring intrinsic resistance that makes a reported result impossible.
Candidates memorize a few breakpoints and apply them across organism groups. They also forget intrinsic resistance rules (for example, Klebsiella pneumoniae is intrinsically resistant to ampicillin), so a 'susceptible' result is reported where resistance is expected.
What to do instead
For each organism-antibiotic pair, state the CLSI document year and table used, and check the intrinsic resistance table before releasing. If a result contradicts known intrinsic resistance, investigate the identification or the test rather than reporting it.
Interpreting an acid-fast smear as negative without specifying the stain type, magnification, and number of fields examined, and confusing smear positivity with culture positivity.
Candidates conflate the screening smear with the diagnostic culture. They also forget that fluorochrome stains are read at lower magnification with different field counts than Ziehl-Neelsen, so the reported sensitivity differs.
What to do instead
Report smears with the stain, magnification, and fields examined, and state that a negative smear does not rule out mycobacterial disease. Explain that culture remains the reference standard and that smear positivity does not establish species.
Treating a single positive Nocardia culture as a contaminant or, conversely, dismissing the need for extended incubation when the culture is only a few days old.
Nocardia grows slowly and can appear as a dry, chalky colony easily overgrown by faster flora. Candidates also misapply the rule that a single respiratory isolate may be a colonizer, forgetting that Nocardia in a sterile site or an immunocompromised host is almost always significant.
What to do instead
Extend incubation for suspected Nocardia and use selective media such as modified Thayer-Martin or buffered charcoal yeast extract when indicated. Interpret significance by site and host status, and confirm identification with modified acid-fast staining and molecular methods.
Identifying a mold to genus or species from macroscopic colony appearance alone, without a tape preparation or slide culture showing conidia, conidiophores, and hyphal septation.
Colony color and texture are memorable and fast to read, but they overlap heavily across genera and change with medium and age. Candidates who rely on gross morphology misidentify dematiaceous and hyaline molds.
What to do instead
Always describe microscopic features (septation, conidial arrangement, fruiting structures) alongside colony characteristics, and note the medium and incubation temperature. When morphology is ambiguous, request molecular identification rather than guessing.
Reporting a parasite ova-and-parasite result as negative after examining only a single stool specimen, or after examining a concentration but not a permanent stained smear.
Candidates treat one negative exam as definitive. Sensitivity depends on parasite burden and on the number of specimens, and some organisms (for example, Strongyloides and Cryptosporidium) require special methods such as agar plate culture or modified acid-fast staining.
What to do instead
State the number of specimens examined and the methods used. Recommend at least three specimens collected on separate days for suspected intestinal parasites, and add the specific method (permanent stain, modified acid-fast, agar plate) when the clinical picture suggests it.
Interpreting a molecular assay result without considering the internal control, the limit of detection, and the possibility of contamination, and treating a positive nucleic acid test as proof of viable organism.
Candidates equate a positive PCR with active infection. They overlook that molecular tests detect nucleic acid, not viability, and that a failed internal control invalidates a negative result.
What to do instead
Before interpreting, confirm the internal control amplified. Distinguish detection of nucleic acid from evidence of viable or replicating organism, and discuss contamination controls and the assay's limit of detection when results are unexpected.
Judging quality control acceptable from a single level or from a Levey-Jennings chart without applying Westgard multirule logic, and failing to document corrective action after a rejection.
Candidates memorize rule names but do not apply them in sequence. They also treat a single out-of-range value as an isolated event rather than checking for a trend or shift, and they omit the documentation step that quality assurance requires.
What to do instead
Work through the rules in order (1-2s warning, then 1-3s, 2-2s, R-4s, 4-1s, 10x) and state the action taken for each rejection. Document the investigation, the corrective action, and the patient impact review, not just the repeat value.
Answering competency and regulatory questions by citing a general principle instead of the specific requirement, such as the required frequency of proficiency testing review or the elements of a documented training record.
Candidates study content domains deeply and treat laboratory management as common sense. They give plausible but unspecific answers that omit the frequency, the responsible person, and the record retention requirement.
What to do instead
For each management topic, memorize the concrete elements: who performs the activity, how often, what is documented, and how long records are retained. Practice writing answers that name the requirement rather than describing good practice in general.