blotting, showing an IgM response, high IgG titer, or seroconversion between acute and convalescent sera, can be used for diagnosis. Molecular tests, such as polymerase chain reaction for DNA or RNA, offer the specificity of culture, high sensitivity, and rapid results. When an unusual infection is suspected, a microbiologist should be consulted before samples are obtained.
The choice of diagnostic imaging mode should be based on the location of the findings. In the absence of localizing signs and during an acute infection, imaging of the entire body is less productive. Plain x-rays are useful initial tests for respiratory tract infections. Ultrasonography is a noninvasive, nonirradiating technique well suited to infants and children for imaging solid organs. It also is useful to identify soft tissue abscesses with lymphadenitis and to diagnose suppurative arthritis of the hip. Computed tomography (CT) (with contrast enhancement) and magnetic resonance imaging (MRI) (with gadolinium enhancement) allow characterization of lesions and precise anatomic localization and are the modalities of choice for the brain. CT shows greater bone detail, and MRI shows greater tissue detail. MRI is especially useful for diagnosis of osteomyelitis, myositis, and necrotizing fasciitis. High-resolution CT is useful for complicated chest infections. Judicious use of CT scans is important because of the longterm effects of radiation on children’s health. Contrast studies (upper gastrointestinal series, barium enema) are used to identify mucosal lesions of the gastrointestinal tract, whereas CT or MRI is preferred for evaluation of appendicitis and intra-abdominal masses. A voiding cystourethrogram may be used to evaluate for vesicoureteral reflux, a predisposing factor for upper urinary tract infections. Radionuclide scans, such as technetium-99m for osteomyelitis and dimercaptosuccinic acid for acute pyelonephritis, are often informative.
Childhood immunization has markedly reduced the impact of major infectious diseases. Active immunization induces immunity through the administration of a vaccine or toxoid (inactivated toxin). Passive immunization includes transplacental transfer of maternal antibodies and the administration of antibody, either as immunoglobulin or monoclonal antibody.
Vaccinations may be with live attenuated viruses (measles, mumps, rubella [MMR], varicella, nasal influenza), inactivated or killed viruses (polio, hepatitis A, intramuscular influenza), recombinant products (hepatitis B, human papillomavirus), reassortants (rotavirus), or immunogenic components of bacteria (pertussis, Haemophilus influenzae type b, Neisseria meningitidis, and Streptococcus pneumoniae), including toxoids (diphtheria, tetanus). Many purified polysaccharides are T-independent antigens that initiate B-cell proliferation without involvement of CD4 T lymphocytes and are poor immunogens in children younger than 2 years of age. Conjugation of a polysaccharide to a protein carrier induces a T-dependent response in infants and creates immunogenic vaccines for
H. influenzae type b, S. pneumoniae, and N. meningitidis.