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Chapter 14. Infectious Diseases: II. Diagnosis of Infectious Diseases

The aims of diagnosis in infectious diseases are (1) to recognize that a sick patient is ill with an infectious disease and (2) to identify the specific etiologic agent in order to determine the most appropriate treatment, as the susceptibility of most pathogens to selected antimicrobial agents is known.

Identification of the infectious agent may be achieved in several ways.

Clinical History

Prevalence of Infectious Diseases

Hospital-Acquired (Nosocomial) Infections

Of the approximately 5 million cases of infectious diseases seen in hospitals annually in the United States, about 40% are acquired in hospitals (nosocomial infections). Hospital-acquired infections are different from those acquired in the community outside the hospital (Table 14-1). The genitourinary tract, surgical wounds, and lungs are the most common sites of nosocomial infection. Most nosocomial infections are caused by bacteria and fungi; viruses, protozoa, and metazoa rarely cause such infections. Hospital-acquired pneumonia is the most serious such infection, with a mortality rate of 20%. Disseminated infections caused by bacteria and fungi, mainly in immunocompromised patients, also account for a significant number of nosocomial infections. In newborn nurseries, hospital-acquired epidemic diarrhea is a common problem.

Table 14–1. Common Infections Seen in Hospitals in the United States. 1
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Table 14–1. Common Infections Seen in Hospitals in the United States. 1
Infection Community-Acquired Hospital-Acquired
Number of cases per year 3 million 2 million
According to type of agent
Bacteria 93% 90%
Viruses 6% Rare
Fungi <1% 10%
Parasites (protozoa, metazoa) <1% Rare
According to site
Genitourinary tract 2 38% 41%
Lung 20% 12%
Upper respiratory tract 8% <1%
Gastrointestinal tract 3% <1%
Central nervous system <1% <1%
Disseminated infection 3 <1% 9%
Others 29% 4 37% 5

1 Note that this table represents only those infections serious enough to warrant hospitalization. The pattern of infectious diseases encountered in general medical practice outside hospitals differs greatly, with mild viral infections of the upper respiratory and gastrointestinal tracts accounting for a great number and percentage of cases.

2 Common genitourinary tract infections in the community include sexually transmitted infections such as gonorrhea.

3 Disseminated infections, commonly caused by bacteria and fungi, occur in immunocompromised patients, usually those being treated for cancer.

4 Includes a wide variety of other sites.

5 Incudes infections of surgical wounds, which are responsible for a majority of this group of hospital-acquired infections.

The high incidence of hospital-acquired infections is due to the following factors:

Increased Susceptibility

Hospitalized patients demonstrate increased susceptibility to infection as a consequence of debilitation associated with concurrent illness and in some cases the administration of immunosuppressive drugs.

Use of Invasive Procedures

Invasive surgical, diagnostic (eg, lumbar puncture, phlebotomy), and therapeutic procedures (eg, bladder catheterization, indwelling intravascular ...

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