Final answer:
A blood test is the primary diagnostic tool for detecting an infection in a joint, as it allows for the identification of pathogens and the determination of appropriate antimicrobial treatment.
Step-by-step explanation:
The diagnostic test that primarily detects the presence of an infection in a joint is a blood test. Diagnosis of infectious arthritis requires the aspiration of synovial fluid from the joint, followed by microscopic evaluation, culture, antimicrobial susceptibility testing, and polymerase chain reaction (PCR) analyses to identify the potential pathogen.
While other imaging studies like X-rays, MRI, and ultrasound can provide images of soft tissues and bones or monitor other conditions such as pregnancies, blood tests are critical for identifying infections and directing treatment with appropriate antimicrobial drugs based on the pathogens identified.