Gastroenteritis 2: Investigations

Biochemical
Stool Microscopy and Culture
Urine Tests
When to perform septic work‐up in dehydrated child with gastro
Taking a blood culture

Blood tests (and other investigations) should NOT be done routinely – but only after due consideration

Each test adds to the cost of the admission – it is a waste of resources to do a test that will not add to the child’s welfare. (Consider: Dr time, appropriate use of resources, child’s discomfort, delays for results etc.) The mainstay of gastroenteritis management is frequent clinical re‐assessment and adjustment of management accordingly.

Biochemical

Biochemical tests should only be done in the following circumstances and only after rehydration has been commenced

Stool Microscopy and Culture

Most diarrhoea is caused by viruses or E.coli which cannot be routinely cultured from the stool. Therefore MC&S should ONLY be done in the following circumstances:

• Dysentery
• Persistent diarrhoea (>2 weeks), especially if HIV positive
• Study of epidemic/outbreak (microbiology input)
• Severe malnutrition

Note 

Antibiotics are only routinely used in dysenteric forms of diarrhoea

Urine Tests

Dipstix testing of bag specimens commonly gives false positive results for possible infection. A negative result is very helpful in excluding infection. Dipstix is therefore not routine but should be done when there is a reasonable likelihood of a UTI:

• Suspected Sepsis with unexplained fever >38 degrees (see protocol)
• Prolonged diarrhoea (>1 week)
• Failure to thrive

If dipstix on a bag specimen is suspicious of a UTI (nitrites/leukocytes), the test must be repeated on a catheter or suprapubic specimen (see UTI protocol). Only if the dipstick on this specimen suggests infection should the sample be sent for microscopy and culture. Do not send bag specimens for MC&S. 

Note

Most suprapubic punctures will be dry in dehydrated children

When to perform septic work‐up in dehydrated child with gastro

a) Child sicker than can be accounted for by dehydration alone:

• urine dipstix (culture if abn dipstix) plus FBC and blood culture 
• consider CSF (may need to be delayed)

b) Infant under 3 months of age:

• urine dipstix, bld culture, FBC, CXR, CRP if<1months and some <3month

Note: CSF must be done in any child with gastro <1month and some <3months (but never perform LP in shocked/unstable child – delay and cover with appropriate antibiotics)

c) Unexplained fever>38.5 degrees:

• urine dipstix (culture if abn dipstix) plus FBC and blood culture

d) persistent or recurrent circulatory shock:

• urine dipstix (culture if abn dipstix) plus FBC and blood culture

e) Severe acute malnutrition (see SAM protocol)

f) Unwell child with dysentery

Note

Children with HIV infection do not require investigations for infections unless they fulfil the above  criteria

Note

CRP is not usually helpful in decision making in these circumstances so should be reserved for infants under 3 months or unexplained source of temperature in very ill child (give antibiotics if  meet the indications below)

Also see:  When to prescribe antibiotics and Taking a blood culture.

Taking a blood culture

The younger and sicker the child, the more useful is a positive or negative blood culture result.

However a blood culture is only indicated in gastroenteritis if:

• < 3 months of age and pyrexial/ill
• <1 month old (by definition a neonate and diarrhoea could indicate parenteral sepsis)
• suspected bacterial meningitis
• Severe acute malnutrition
• Older “septic‐looking” very unwell/pyrexial child e.g. with dysentery

Blood cultures MUST be taken with aseptic technique, with a minimum volume of 1ml in a neonate and 3‐5ml in older child to increase the yield.

Always consider if the blood culture result might alter management and remember the dangers of false positives in terms of cost in lab, unnecessary exposure to antibiotics and prolonging hospital stay.

This guidance is in part taken from:

WC Gastroenteritis Fam Med & Paeds Protocol 2014 &

New Somerset Hosp Paeds Gastro Protocol 2014