It would be expected that the time of admission to the intensive care unit would occur after the time of admission to the hospital. Sometimes, however, admission to the intensive care unit occurs before admission to the hospital. How is this explained?
For around 61046 admissions out of the 61532 admissions (~99.2% admissions), the ICU intime occurs after the hospital admission time.
SELECT i.intime - a.admittime as tdiff,
a.subject_id, a.hadm_id, a.admittime, i.intime,
FROM mimiciii.admissions a
INNER JOIN mimiciii.icustays i
ON a.hadm_id = i.hadm_id
ORDER BY i.intime - a.admittime desc;
For the remaining proportion of records where admission to the ICU is recorded as occuring before admission to the hospital, the time differences are generally very small.
In all but 41 admissions the ICU intime occurs no more than 12 hours before time of admission to the hospital. In all but 2 admissions the ICU intime occurs no more than 24 hours before the time of admission to the hospital.
In principle hospital admission time should not occur before ICU intime, but in some cases it is possible that administration delays or errors lead to date mismatches such as these.