Chapter 35: Antimicrobial Regimen Selection

INTRODUCTION

• A systematic approach to the selection and evaluation of an antimicrobial regimen is shown in Table 35-1. An empiric antimicrobial regimen is begun before the offending organism is identified and sometimes before documentation of the presence of infection, whereas a definitive regimen is instituted when the causative organism is known.

CONFIRMING THE PRESENCE OF INFECTION

Fever

• Fever is defined as a controlled elevation of body temperature above the expected 37°C (98.6°F) (measured orally) and is a manifestation of many disease states other than infection.

• Many drugs have been identified as causes of fever. Drug-induced fever is defined as persistent fever in the absence of infection or other underlying condition. The fever must coincide temporally with the administration of the offending agent and disappear promptly upon its withdrawal, after which the temperature remains normal.

White Blood Cell Count

• Most infections result in elevated white blood cell (WBC) counts (leukocytosis) because of the mobilization of granulocytes and/or lymphocytes to destroy invading microbes. Normal values for WBC counts are between 4000 and 10,000 cells/mm³ (4 × 10⁹ and 10 × 10⁹/L).

• Bacterial infections are associated with elevated granulocyte counts (neutrophils and basophils), often with increased numbers of immature forms (band neutrophils) seen in peripheral blood smears. With infection, peripheral leukocyte counts may be high, but they are rarely higher than 10,000 to 40,000 cells/mm³ (10 × 10⁹/L to 40 × 10⁹/L). Low neutrophil counts (neutropenia) after the onset of infection indicate an abnormal response and are generally associated with a poor prognosis for bacterial infection.

• Relative lymphocytosis, even with normal or slightly elevated total WBC counts, is generally associated with tuberculosis and viral or fungal infections. Many types of infections, however, may be accompanied by a completely normal WBC count and differential.

Local Signs

• Pain and inflammation may accompany infection and are sometimes manifested by swelling, erythema, tenderness, and purulent drainage. Unfortunately, these signs may be apparent only if the infection is superficial or in a bone or joint.

• The manifestations of inflammation with deep-seated infections such as meningitis, pneumonia, endocarditis, and urinary tract infection must be ascertained by examining tissues or fluids. For example, the presence of polymorphonuclear leukocytes (neutrophils) in spinal fluid, lung secretions (sputum), and urine ...