MLS Clinical Testing & Analysis 3 — Questions and Answers
Question 1: A type and screen is ordered pre-operatively. The antibody screen is positive. What is the next step?
- Perform antibody identification panel (Correct answer)
- Immediately issue O-negative blood
- Repeat the ABO/Rh typing only
- Cancel the crossmatch
Correct answer: Perform antibody identification panel
A positive antibody screen requires an antibody identification panel to determine the specificity of the unexpected antibody before blood is issued.
Question 2: In cerebrospinal fluid analysis, xanthochromia is most commonly caused by:
- Oxyhemoglobin and bilirubin from red cell breakdown (Correct answer)
- Elevated CSF protein alone
- Bacterial meningitis pigments
- Traumatic tap artifact
Correct answer: Oxyhemoglobin and bilirubin from red cell breakdown
Xanthochromia results from oxyhemoglobin and later bilirubin released as RBCs lyse after subarachnoid hemorrhage, distinguishing it from a traumatic tap.
Question 3: Which mycobacterium is the most common cause of pulmonary tuberculosis in immunocompetent US adults?
- Mycobacterium tuberculosis (Correct answer)
- Mycobacterium avium complex
- Mycobacterium kansasii
- Mycobacterium fortuitum
Correct answer: Mycobacterium tuberculosis
Mycobacterium tuberculosis is the etiologic agent of classic tuberculosis and remains the leading mycobacterial pathogen in immunocompetent individuals.
Question 4: A serum potassium is critically elevated. The specimen is hemolyzed. What should the laboratory do?
- Reject the specimen and request a recollect (Correct answer)
- Report the value with a hemolysis comment
- Dilute the specimen and correct mathematically
- Run the specimen on a different analyzer
Correct answer: Reject the specimen and request a recollect
Hemolysis releases intracellular potassium, falsely elevating serum K⁺; the specimen must be rejected and a fresh non-hemolyzed sample collected for accurate results.
Question 5: Which coagulation test is used to monitor unfractionated heparin therapy?
- Activated partial thromboplastin time (aPTT) (Correct answer)
- Prothrombin time (PT)
- Thrombin time only
- Platelet aggregation studies
Correct answer: Activated partial thromboplastin time (aPTT)
Unfractionated heparin potentiates antithrombin to inhibit intrinsic pathway factors, which is measured by the aPTT; therapeutic range is typically 1.5–2.5× control.
Question 6: On a peripheral smear, which cell morphology is most characteristic of iron-deficiency anemia?
- Microcytic hypochromic cells with pencil cells (Correct answer)
- Macrocytic oval cells
- Target cells and sickle cells
- Spherocytes with polychromasia
Correct answer: Microcytic hypochromic cells with pencil cells
Iron deficiency produces small, pale RBCs (microcytic/hypochromic) with elongated pencil cells due to reduced hemoglobin synthesis.
Question 7: A urine culture grows >100,000 CFU/mL of a lactose-fermenting gram-negative rod. Oxidase is negative. Most likely organism is:
- Escherichia coli (Correct answer)
- Pseudomonas aeruginosa
- Klebsiella pneumoniae
- Proteus mirabilis
Correct answer: Escherichia coli
E. coli is oxidase-negative, lactose-fermenting, and is the most common cause of community-acquired urinary tract infections at significant colony counts.
MLS Clinical Testing & Analysis 3
A type and screen is ordered pre-operatively.
The antibody screen is positive.
What is the next step?