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Volume 2 Issue 4 April 1997

Acute gastroenteritis caused by rotavirus is highly seasonal occurring in Kansas City almost exclusively during the winter and spring months. The percent of positive antigen tests at The Children’s Mercy Hospital are shown below. The disease starts first in the fall in the southwest region of the United States and gradually spreads to the northeast by early spring. Although the onset is geographically gradual, the cessation of the disease is abrupt and occurs during the summer simultaneously throughout the country. Using the “gold standard” of electron microscopic examination of the stool, the CDC determined that in the southwest there is an endemic level of rotavirus disease in children with diarrhea even in the summer months. However, in other parts of the country a positive test in the summer is more likely to be a false positive than a true positive. This may mislead the clinician into believing that the patient has a benign disease when, in fact, there may be serious possibilities. Last May, the tail end of rotavirus season in Kansas City, a case of diarrhea was diagnosed as being caused by rotavirus which, only after significant additional morbidity, proved to be associated with C. difficile. In 1996 The Children’s Mercy Hospital initiated the policy of requiring clinical pathology consultation prior to testing during the “off season” in an attempt to prevent this problem.

	1994	1995	1996
January	38 (36/96)	33 (16/49)	38 (13/34)
February	37 (29/78)	51 (78/154)	59 (29/49)
March	25 (21/83)	44 (68/155)	39 (28/72)
April	17 (11/64)	21 (10/48)	27 (10/39)
May	3 (1/31)	8 (3/39)	0 (0/9) (1st week)
July	0 (0/13)	5 (1/22)
August	0 (0/20)	0 (0/9)	test not available
September	5 (2/44)	5 (1/22)	test not available
October	0 (0/17)	5 (1/22)
November	0 (0/26)	1 (1/27)	4 (1/28)
December	0 (0/30)	10 (5/48)	5 (1/22)

The “gold standard” for the diagnosis of rotavirus is electron microscopic examination of stool, a procedure which will not be done. Therefore, it is necessary to assume that the percent of positive antigen tests is an estimate of the prevalence of the disease in the community. The Positive Predictive Value (PPV) of a test, defined as the percent of patients with positive test results who actually have the disease, changes markedly with a change in prevalence. In this community, when the prevalence of rotavirus diarrhea is below 5%, a positive test is more likely to be a false positive than a true positive.

The significance of a false positive test result depends on the clinical setting. For example, when a test is used for screening, particularly for a serious, treatable disease, a relatively large number of false positives may be acceptable. Diagnosis will be confirmed by other means and the danger is in missing a case (i.e. a false negative). Rotavirus disease is a benign disease which is only treated symptomatically with rehydration. Treatment is not based on identifying the etiological agent, but on clinical assessment of the hydration status or perhaps an electrolyte test. Failure to identify the etiological agent is not dangerous. However, if a positive test falsely reassures the clinician that the patient has a benign, self-limiting disease and delays further diagnostic workup, that may be dangerous. During the summer months the rotavirus test may do just that and should only be used with that understanding.

References:

Lebaron, CW et al. Outbreaks of Summer Rotavirus Linked to Laboratory Practices. Pediatr Infect Dis J: 1992:860-5.