Acute diarrhea — looser, more liquid or more frequent stools than usual for less than 7 days — is diagnosed from the history and the examination, and a child with an uncomplicated illness needs no test at all. The useful question is therefore not what could be measured, but which result would change the plan: an organism that would be treated, a sodium concentration that would change the rate at which fluid is given, or an alternative diagnosis that the illness does not explain.
When a stool specimen is worth sending
Microbiological investigation is not routine and usually does not change treatment, because most illness is self-limiting whatever the organism and the decision to give an antibiotic rests on the clinical picture. It is worth considering in a smaller set of situations:
- a child who is extremely unwell, or in whom septicemia (bloodstream infection with systemic illness) is suspected;
- blood or mucus in the stool, and in particular severe bloody diarrhea;
- recent travel to an at-risk area, or possible exposure to cholera;
- an underlying chronic condition or immune deficiency, in which the pathogen may be one that needs specific treatment;
- diarrhea that has not improved by day 7, or that is uncertain enough that another diagnosis is being considered.
In most of these situations the organism’s identity could change what is done for the child; in the last, the diagnosis of acute gastroenteritis is itself in doubt.
One result carries a consequence beyond treatment. When Shiga toxin-producing E. coli (STEC) is identified, the child needs monitoring for hemolytic uremic syndrome — a rare complication in which damaged small blood vessels destroy red cells, the platelet count falls and the kidneys are injured — rather than empirical antibiotics, and the features to watch for are described in Complications and Outcome of Acute Diarrhea and Dehydration.
Stool sampling during an outbreak serves a different purpose. In a day-care center, school, hospital or residential setting, identifying the organism and its source is what allows the outbreak to be controlled, so a specimen there is sent for public health reasons rather than for the individual child.
Markers that do not separate bacterial from viral
Inflammatory markers have been tested as a shortcut to the bacterial-or-viral question. CRP (C-reactive protein, an acute-phase protein that rises with inflammation) and procalcitonin, another blood marker, are not recommended for distinguishing a bacterial from a viral cause. A raised CRP can be found in bacterial gastroenteritis and a normal CRP does not exclude it, so the result does not settle the question that prompted it; the studies that tested these markers were not of high quality either. Fecal markers of intestinal inflammation such as calprotectin and lactoferrin are not recommended for this purpose at all, having been developed for inflammatory bowel disease and being raised in many conditions.
The distinction is not pursued with tests because it rarely changes treatment. Most gastroenteritis in an otherwise healthy child is self-limited whichever organism is responsible, and antibiotics are reserved for defined pathogens and defined children, as set out in Refeeding and Drug Therapy in Acute Diarrhea.
When to measure electrolytes and glucose
Blood biochemistry is not part of the routine assessment of gastroenteritis. It becomes useful in a small number of situations, and they share the same shape: a measurement that would change the rate, the composition or the route of fluid therapy.
- Intravenous fluid is being started. Electrolytes are measured in every child on intravenous therapy and during that therapy, because hyper- or hyponatremia alters the rate at which fluid may be given. Plasma sodium, potassium, urea, creatinine and glucose are the measurements taken, and venous acid–base status and chloride are added when shock is suspected or confirmed.
- The dehydration does not fit the story. Electrolytes are also measured in moderately dehydrated children whose history and examination are inconsistent with the severity of the diarrheal illness, and in all severely dehydrated children.
- A feature suggests hypernatremia. Jittery movements, increased muscle tone, hyperreflexia, convulsions, drowsiness or coma make the sodium measurement urgent rather than optional, because the degree of dehydration is easily underestimated in this group.
A normal serum bicarbonate lowers the likelihood of a deficit above 5%, but no laboratory test so far can estimate the percentage of weight lost in general practice, and none of these measurements replaces the severity grade.
What the results feed
Each measurement answers a narrow question and nothing more. The sodium concentration decides how fast a deficit may be corrected, which is the arithmetic of Rehydration Management in Acute Diarrhea. Recognising the sodium pattern before the result returns is a clinical skill, part of Clinical Assessment of Acute Diarrhea and Dehydration. And the decision to escalate care is triggered more often by the child’s appearance and course than by a test, as the criteria in Admission and Shock Management in Acute Diarrhea show.