Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (511 page)

Culture
: Culture is the gold standard for diagnosis of amebiasis, but it is not widely available.

Direct detection
: Detection of trophozoites or cysts in stool is the most common diagnostic procedure. The sensitivity of a single stool specimen is <50%. At least three samples, collected on consecutive days, should be examined before ruling out amebiasis. The observation of phagocytized RBCs is specific for
E. histolytica
and provides differentiation from
E. dispar
. Motile trophozoites may be detected in saline wet mounts if stool can be examined immediately. The finding of many RBCs, but minimal WBCs, on microscopic examination of stool helps to differentiate amebiasis from bacillary dysentery.

Serology and antigen testing
: The indirect hemagglutination assay for
E. histolytica
antibody is 99% sensitive in patients with liver abscess and 88% sensitive for intestinal disease. Tests remain positive for years and cannot distinguish acute from past infection. Detection of stool antigen is sensitive (95%) and specific (93%) for
E. histolytica
.

Histology
: Endoscopic biopsy or smear of exudate of sigmoid ulcers may show
E. histolytica
in 50% of cases. Collect from six or more lesions for permanent staining. Tissue diagnosis for amebic liver abscess is rarely performed; imaging studies and serologic and antigen studies can usually confirm this diagnosis. When sampled, amoebae are usually located in the abscess wall, not in the necrotic contents of the abscess. Parasites are identified in abscess material in <20% of cases.

Core laboratory
: Liver abscess should be suspected in patients with risk factors who present with fever (90%), leukocytosis, increased ALP, and right upper quadrant pain and tenderness (85%). The right hemidiaphragm may be elevated. Many patients (60%) with liver abscess have no history of intestinal disease; stool for O&P is positive in <20–40% of patients with hepatic abscess. Eosinophilia is uncommon.

ASCARIASIS (
ASCARIS LUMBRICOIDES
)

   Definition

Ascaris lumbricoides
is a large intestinal roundworm with a global distribution. After ingestion, embryonated eggs hatch, releasing second-stage larvae in the intestinal lumen. These penetrate into the capillaries and lymphatics of the intestinal mucosa. From the circulation, they are deposited in the lungs where they develop into fourth-stage larvae. Fourth-stage larvae migrate up the trachea and are swallowed, returning to the small intestine, where they develop into mature adults.

   Who Should Be Suspected?

Most infections are asymptomatic, but mild, nonspecific pulmonary or abdominal symptoms may occur. Symptoms may be caused by immune response, effects of larval migration, large worm burden, and nutritional impact. Pneumonitis may occur (e.g., Loeffler syndrome) during migration. With high worm burden, malnutrition or intestinal, biliary, or pancreatic obstruction may occur. Nausea, vomiting, diarrhea, and other conditions may develop.

   Laboratory Findings

Direct detection
: Identification of eggs by routine O&P examination is the usual method of identification. Larvae are occasionally seen in sputum or gastric aspirates. In pneumonitis associated with a primary infection, stool examination for eggs may be negative.

Radiology
: Abnormalities associated with pneumonitis may be transient.

Core laboratory
: Eosinophilic reaction is common during symptomatic disease.

BABESIOSIS

   Definition

Babesia microti
, a protozoan blood parasite, is transmitted by the tick
Ixodes scapularis
, which is also the vector for Lyme borreliosis and human granulocytic ehrlichiosis. Sexual reproduction occurs in the tick. After the infective forms enter the host during a blood meal, they enter erythrocytes, where asexual reproduction occurs. Most cases of babesiosis occur in the Northeastern and Great Lakes states in the United States and are caused by
Babesia microti
. Other
Babesia
species cause infections in other regions of the United States as well as in Europe, and these infections may differ in their clinical presentation. Transmission of babesiosis by transfusion is well described.

   Who Should Be Suspected?

   Most
Babesia
infections are likely to be asymptomatic or subclinical. In symptomatic infections, influenza-like symptoms with fever, accompanied by sweats and chills, malaise, fatigue, weakness, and joint pain are seen with onset in the first month after tick bite or 1–2 months after transmission by transfusion. Fever and severe symptoms generally resolve within several weeks, but milder malaise and fatigue may linger for months.

   Severe disease can occur in patients with asplenia or other immunocompromising condition. These patients may develop very high parasitemia levels, leading to hemolysis and jaundice, anemia, renal failure, DIC, ARDS, hypotension, and other complications.

   Infections caused by
Babesia divergens
almost always occur in splenectomized patients. Disease is rapidly progressive and severe and is associated with high mortality. After an incubation period of 1–4 months, high fevers, malaise, myalgias, headache, hypotension, jaundice, intravascular hemolysis, and renal failure may occur. Diarrhea, nausea, and vomiting are prominent symptoms. Coma and death occur within 1 week after onset of symptoms in almost 50% of patients.

   Laboratory Findings