Acute diarrhea and acute colitis of infectious origin are common reasons for consultation at the emergency department. The current etiological diagnostic approach is limited to the determination of markers of inflammation, such as CRP and blood leukocytes, which lack specificity and sensitivity for bacterial infection. The stool culture can detect bacterial pathogens in the stool with a result at least 48 hours later and a positivity rate <50%.

This study will describe the procalcitonin (PCT) concentrations (a biomarker of bacterial infection) in this population to evaluate its usefulness depending on the viral or bacterial etiology identified by stool multiplex gastro-intestinal PCR panel (GI panel) and stool culture.

The investigators hypothesize that PCT levels will be higher if the GI panel or the stool culture identifies a bacteria or a parasite, as it is the case in respiratory tract infections. If there is a detection of a virus by the GI panel or both the stool culture and the GI panel are negative, the investigators expect that PCT values will be lower or negative.

the investigators will include the patients admitted to the ED with a suspicion of infectious diarrhea or acute colitis in order to have a large representative panel of infectious diarrhea etiologies.

Only the patients having a blood sample prescribed as the routine care will be included. The blood sample is useful for dosing CRP and whole blood cell count (WBC), which are part of current biologic analyses performed in this context. After getting the patient's consent, the investigator will add the PCT dosage in blood sampling and will ask the patient to provide a stool sample, in order to have a stool culture and to perform an extended investigation for the pathogens through multiplex PCR technology (Filmarray ®GI panel).

The physician will be asked if all these results (the ones ordered currently together with the dosage of PCT and the GI panel) will change his/her decision to start an antibiotic.

Patients will receive a phone call at day 15 after their initial admission in the emergency department and will be asked if he/she has consulted a new physician or if a new treatment by antibiotics was started.

Data collection procedures: Data from the medical file will be collected by the investigators and the emergency department clinical research assistant. All the data will be pseudonymized. The collection will be done at the day of admission in the emergency department and after the phone interview at Day15.

Acute diarrhea and acute colitis of infectious origin are common reasons for consultation at the emergency department. The current etiological diagnostic approach is limited to the determination of markers of inflammation, such as CRP and blood leukocytes, which lack specificity and sensitivity for bacterial infection. The stool culture can detect bacterial pathogens in the stool with a result at least 48 hours later and a positivity rate <50%.

This study will describe the procalcitonin (PCT) concentrations (a biomarker of bacterial infection) in this population to evaluate its usefulness depending on the viral or bacterial etiology identified by stool multiplex gastro-intestinal PCR panel (GI panel) and stool culture.

The investigators hypothesize that PCT levels will be higher if the GI panel or the stool culture identifies a bacteria or a parasite, as it is the case in respiratory tract infections. If there is a detection of a virus by the GI panel or both the stool culture and the GI panel are negative, the investigators expect that PCT values will be lower or negative.

The investigators will include the patients admitted to the ED with a suspicion of infectious diarrhea or acute colitis in order to have a large representative panel of infectious diarrhea etiologies.

Only the patients having a blood sample prescribed as the routine care will be included. The blood sample is useful for dosing CRP and whole blood cell count (WBC), which are part of current biologic analyses performed in this context. After getting the patient's consent, the investigator will add the PCT dosage in blood sampling and will ask the patient to provide a stool sample, in order to have a stool culture and to perform an extended investigation for the pathogens through multiplex PCR technology (Filmarray ®GI panel).

The physician will be asked if all these results (the ones ordered currently together with the dosage of PCT and the GI panel) will change his/her decision to start an antibiotic.

Patients will receive a phone call at day 15 after their initial admission in the emergency department and will be asked if he/she has consulted a new physician or if a new treatment by antibiotics was started.

Data collection procedures: Data from the medical file will be collected by the investigators and the emergency department clinical research assistant. All the data will be pseudonymized. The collection will be done at the day of admission in the emergency department and after the phone interview at Day15.

Statistical justification for sample size: This will be a pilot study to describe the results of PCT concentration and multiplex GI panel. As to our knowledge, there is no published study reporting PCT concentrations in this population. This preclude a sample, size calculation. The investigators will include as much patients as possible during one year in order to cover the epidemiological seasonal variations and to obtain a convenient sample of at least 100 patients.

Statistical methods description: Clinical and biological data will be reported as median and interquartile range or mean +/- SD. PCT values will be reported as median and IQR, respectively if a there is a viral infection, a documented infection (bacterium or parasite) or no pathogen identified by GI panel and/or stool culture (bacteria). Patient's characteristics and PCT results will be compared in all groups, using as a reference, the group where no pathogen is observed. A sub-group analysis will be performed on colitis and acute diarrhea respectively, to find out if there is any difference with the variables analyzed as both pathologies are not managed in the same way.

Inclusion Criteria:

Inclusion criteria for patients presenting an acute diarrhoea :

1. Patients over 18 years of age,
2. Signed informed consent form
3. Having a social security insurance,
4. Attending the emergency department for Acute diarrhoea (at least three loose or liquid stools a day for less than 15 days)
5. For whom a blood test in the emergency department is ordered as a standard of care

Inclusion criteria for patients presenting an infectious colitis :

1. Patients over 18 years of age,

2. Signed informed consent form

3. Having a social security insurance,

4. Attending the emergency department for a documented colitis presumed to be of infectious origin, for whom a blood test in the emergency department is planned, defined by:

   1. at least one of the following symptoms: abdominal pain, fever, diarrhoea and
   2. Elevated white blood cell count (> 10.000/mm3) or CRP (> 10 mg/l) and
   3. An abdominal CT scan showing a thickening of at least segmental colonic walls.

Exclusion Criteria:

1. Recent abdominal surgery of less than one month
2. Pregnancy
3. Inflammatory bowel disease
4. Patients under curatorship or guardianship