Diagnostic Procedures Flashcards

Which test differentiates Staphylococcus species from Streptococcus species?

Answer: Catalase

Staphylococci produce catalase, which breaks down hydrogen peroxide and makes bubbles, while streptococci are catalase-negative.

A positive tube coagulase test on a gram-positive coccus in clusters identifies which organism?

Answer: Staphylococcus aureus

Free coagulase converts fibrinogen to fibrin and is the classic marker for S. aureus among common clinical staphylococci.

An abnormal PT corrects completely when patient plasma is mixed 1:1 with normal pooled plasma. This most likely indicates what?

Answer: A factor deficiency

Normal plasma supplies the missing factor and corrects a deficiency, whereas an inhibitor would also neutralize the normal plasma.

In a sickle cell solubility screening test, what causes a positive turbid result?

Answer: Reduced HbS polymerizes and becomes insoluble in the phosphate buffer

Sodium dithionite deoxygenates hemoglobin, and deoxy-HbS is insoluble and makes the solution cloudy so that lines behind the tube cannot be seen.

Red blood cell casts in a urine sediment are most indicative of what?

Answer: Glomerulonephritis

RBC casts show that bleeding comes from the glomerulus or tubules, which is classic for glomerular disease.

Which laboratory method is used to enumerate CD4+ T lymphocytes for monitoring HIV infection?

Answer: Flow cytometry

Flow cytometry uses fluorescent-labeled monoclonal antibodies to identify and count cells that express CD4.

How long should a patient typically fast before a lipid panel that includes triglycerides?

Answer: 9 to 12 hours

Triglycerides rise after meals, so a 9-12 hour fast is standard when an accurate triglyceride or calculated LDL is needed.