MLT GlobeASCP exam preparation (USA · MLS / MLT) – page 53
1200 practice MCQs for the ASCP medical laboratory exam. Level: Advanced.
Arterial blood has bicarbonate 24 mmol/L and pCO2 40 mm Hg. Using pH = 6.1 + log [HCO3⁻/(0.03 × pCO2)], the pH is:
0.03 × 40 = 1.2 mmol/L; 24/1.2 = 20; log 20 = 1.30; 6.1 + 1.30 = 7.40.
How many grams of calcium chloride (CaCl2, MW 111 g/mol) are needed to prepare 1 L of a solution containing 100 mEq/L?
Ca²⁺ has valence 2, so 100 mEq = 50 mmol. 50 mmol × 111 mg/mmol = 5550 mg = 5.55 g. 11.1 g treats mEq as mmol.
A WBC count uses a 1:20 dilution. A total of 100 cells are counted in the four large corner squares (each 1 mm², depth 0.1 mm). The WBC count is:
Volume counted = 4 × 1 × 0.1 = 0.4 µL. Cells/µL = (100 × 20)/0.4 = 5000/µL = 5.0 × 10⁹/L.
The OSHA permissible exposure limit for formaldehyde as an 8-hour time-weighted average is:
OSHA 29 CFR 1910.1048 sets the formaldehyde PEL at 0.75 ppm (8-h TWA), a short-term limit of 2 ppm and an action level of 0.5 ppm.
A patient has a hematocrit of 60%. Using C = 1.85 × 10⁻³ × (100 − Hct) × V, how much 3.2% citrate is needed for 4.5 mL of blood?
C = 0.00185 × (100 − 60) × 4.5 = 0.00185 × 40 × 4.5 = 0.333 mL. The standard 0.5 mL would be too much citrate because plasma volume is reduced.
Four consecutive results for one control level are +1.4, +1.6, +1.2 and +1.8 SD. Which Westgard rule is violated?
Four consecutive values beyond the same 1 SD limit violate 4-1s, which signals a small systematic error. No value exceeds 2 SD, so 2-2s is not violated.
Five control results are 98, 100, 102, 100 and 100 mg/dL. What is the sample standard deviation?
Mean = 100. Squared deviations sum to 4+0+4+0+0 = 8. Sample SD = √(8/(n−1)) = √(8/4) = 1.41. Dividing by n (5) gives 1.26, the population SD, which is not used for QC.
A lab verifies a manufacturer's reference interval using 20 healthy local subjects. The interval is accepted if no more than how many results fall outside it?
Under CLSI EP28, if 2 or fewer of 20 results (≤10%) fall outside the proposed limits, the interval can be transferred. If more fall outside, a further 20 are tested or a new interval is set.
Which method validation experiment is designed to estimate proportional systematic error?
In a recovery study, known amounts of analyte are added to patient samples; incomplete recovery that grows with concentration shows proportional error. Interference studies estimate constant systematic error.
An anaerobic gram-negative rod from an abdominal abscess is resistant to vancomycin 5 µg, kanamycin 1 mg and colistin 10 µg special-potency disks. It grows on Bacteroides bile esculin agar with blackening. It belongs to the:
The B. fragilis group resists all three disks, grows in 20% bile and hydrolyzes esculin (black colonies). Fusobacterium is kanamycin and colistin susceptible, and pigmented Prevotella and Porphyromonas are bile sensitive.
An anaerobic gram-positive coccus from a wound is inhibited by a sodium polyanethol sulfonate (SPS) disk. It is most likely:
P. anaerobius is characteristically susceptible to SPS, giving a zone of 12 mm or more. Other anaerobic gram-positive cocci are resistant, and Veillonella is a gram-negative coccus.
A continuously monitored blood culture bottle signals positive, but the Gram stain shows no organisms. What is the most appropriate next step?
Low numbers of organisms, or organisms that stain poorly, may be missed on Gram stain; acridine orange is more sensitive, and subculture confirms growth. Very high white cell counts can also give false signals.
Enterococcus faecalis ATCC 29212 gives a very small zone around trimethoprim-sulfamethoxazole on a new lot of Mueller-Hinton agar. The most likely problem is:
Thymidine lets bacteria bypass folate inhibition, falsely lowering trimethoprim-sulfonamide activity. E. faecalis ATCC 29212 is used to check that thymidine content is acceptably low. pH 7.2–7.4 is the normal range.
Pseudomonas aeruginosa ATCC 27853 shows aminoglycoside zones below the QC range, while other drugs are in range. The most likely medium problem is:
High divalent cation levels reduce aminoglycoside uptake by P. aeruginosa, giving smaller zones (false resistance). Low cations have the opposite effect. Low zinc affects carbapenem testing, not aminoglycosides.
On a gradient diffusion strip, the edge of the ellipse of inhibition meets the strip between the 0.19 and 0.25 µg/mL marks. What MIC should be reported?
Gradient strips have values between standard two-fold dilutions. The reading is taken where the ellipse meets the strip and is rounded up to the next two-fold dilution, 0.25 µg/mL, before interpretation.
A laboratory wants to change routine AST QC from daily to weekly testing. According to CLSI, which result supports this change?
CLSI allows weekly QC after a 15-replicate plan (or the 20/30-day plan) with acceptable results for each drug and QC strain. Five days is too few, and proficiency testing does not replace QC.
A Staphylococcus aureus has a penicillin zone of 30 mm. The edge of the zone is sharp and clear-cut ('cliff') rather than fuzzy ('beach'). How should penicillin be reported?
CLSI recommends the penicillin zone-edge test for S. aureus with large zones. A sharp 'cliff' edge shows beta-lactamase production, so the isolate is reported resistant despite the large zone. A fuzzy 'beach' edge suggests no beta-lactamase.
Which E. coli pathotype is often non-motile, lactose non-fermenting or late, lysine decarboxylase negative, and causes dysentery like Shigella?
EIEC invades colonic epithelium like Shigella and shares many of its biochemical features, which can cause misidentification.
An Enterococcus faecium from a rectal screening swab is highly resistant to both vancomycin and teicoplanin. Which gene is most likely responsible?
vanA gives high-level resistance to both vancomycin and teicoplanin and is the commonest VRE gene. vanB strains usually remain teicoplanin susceptible.
A catalase-negative gram-positive coccus in pairs and short chains is intrinsically vancomycin resistant and produces gas from glucose in MRS broth. It is most likely:
Leuconostoc is intrinsically vancomycin resistant and produces gas from glucose. Pediococcus is also vancomycin resistant but forms tetrads and produces no gas.