Diagnostic Procedures & Reporting Flashcards

Which of the following is the preferred specimen for detecting Clostridioides difficile toxin?

Answer: Unformed (liquid or soft) stool

C. difficile testing should only be performed on unformed stool, as formed stool from asymptomatic carriers gives false-positive results and does not indicate active colitis.

A prothrombin time (PT) is reported as an INR of 3.8 for a patient on warfarin therapy. The therapeutic range for most indications is 2.0–3.0. This result should be:

Answer: Reported as a critical value requiring immediate physician notification

An INR of 3.8 exceeds the typical upper critical threshold (often ≥4.0 or institution-specific), and supratherapeutic anticoagulation poses bleeding risk requiring prompt clinical notification.

A positive DAT (direct antiglobulin test) with anti-IgG reagent, combined with a negative DAT with anti-C3d, is most consistent with:

Answer: Drug-induced immune hemolytic anemia or warm autoimmune hemolytic anemia

Warm AIHA and many drug-induced cases coat red cells with IgG without complement deposition, producing a positive anti-IgG DAT and negative anti-C3d DAT.

A stool culture grows Salmonella typhi. Besides the clinical team, this result must be reported to:

Answer: Public health authorities, as it is a reportable pathogen

Salmonella typhi (typhoid fever) is a nationally notifiable disease in the US; laboratories are required to report positive cultures to local and state public health departments.

In urinalysis, a positive nitrite test indicates the presence of:

Answer: Gram-negative bacteria that reduce nitrate to nitrite

The nitrite test detects bacteria (primarily Gram-negatives like E. coli) that convert dietary nitrate to nitrite via reductase enzymes; many Gram-positives lack this enzyme and produce a negative nitrite result.

A laboratory uses Westgard rules. A 1-3s warning rule is triggered on a quality control chart. The appropriate action is:

Answer: Accept the run but investigate for a possible trend

The 1-3s rule is a warning rule only; the run should be accepted but reviewed for emerging trends, and the 1-2s or multi-rule pattern assessed before rejecting.

Which test is used to confirm a positive screening result for phenylketonuria (PKU) on a newborn dried blood spot?

Answer: Plasma phenylalanine quantitation by tandem mass spectrometry

Confirmatory testing for PKU uses quantitative plasma phenylalanine measurement (typically by LC-MS/MS), which precisely determines phenylalanine levels to guide diagnosis and management.