First ICU Stay Selection

Identifies first ICU stays and hospital admissions for cohort construction. Critical for avoiding correlated observations and inflated sample sizes from readmissions.

When to Use This Skill

• Creating research cohorts with independent observations
• Excluding readmissions that may have different characteristics
• Selecting incident (first-time) ICU patients
• Matching cohorts by admission characteristics

Pre-computed Table

SELECT
    subject_id,
    hadm_id,
    stay_id,
    -- Demographics
    gender,
    dod,  -- Date of death (if applicable)
    -- Hospital admission
    admittime,
    dischtime,
    los_hospital,  -- Hospital length of stay (days)
    admission_age,
    race,
    hospital_expire_flag,
    hospstay_seq,       -- Hospital admission sequence number
    first_hosp_stay,    -- TRUE if first hospitalization
    -- ICU stay
    icu_intime,
    icu_outtime,
    los_icu,            -- ICU length of stay (days)
    icustay_seq,        -- ICU stay sequence within hospitalization
    first_icu_stay      -- TRUE if first ICU stay of this hospitalization
FROM mimiciv_derived.icustay_detail;

Key Columns Explained

Hospital-Level

• hospstay_seq

  : Order of hospital admission for this patient (1 = first ever)

• first_hosp_stay

  : TRUE if this is the patient's first hospital admission

ICU-Level (Within Hospitalization)

• icustay_seq

  : Order of ICU stays within this hospitalization (1 = first ICU stay)

• first_icu_stay

  : TRUE if this is the first ICU stay of the current hospitalization

Age Calculation

Age is calculated as:

anchor_age + (YEAR(admittime) - anchor_year)

Important: MIMIC-IV shifts ages for patients > 89 years old at any admission. These patients have

anchor_age = 91

and ages are not exact.

Common Cohort Selection Patterns

First ICU Stay Only (Most Common)

SELECT *
FROM mimiciv_derived.icustay_detail
WHERE first_icu_stay = TRUE;

First-Ever ICU Stay (Strictest)

SELECT *
FROM mimiciv_derived.icustay_detail
WHERE first_hosp_stay = TRUE
    AND first_icu_stay = TRUE;

Adult Patients Only

SELECT *
FROM mimiciv_derived.icustay_detail
WHERE admission_age >= 18
    AND first_icu_stay = TRUE;

Exclude Very Short Stays

SELECT *
FROM mimiciv_derived.icustay_detail
WHERE first_icu_stay = TRUE
    AND los_icu >= 1  -- At least 24 hours in ICU
    AND los_hospital >= 1;

Critical Implementation Notes

1. Multiple ICU Stays: One hospitalization can have multiple ICU stays.

   first_icu_stay

   identifies the first ICU stay within each hospitalization.

2. Patient-Level vs Stay-Level: Choose the right level of independence:

   • first_hosp_stay = TRUE AND first_icu_stay = TRUE

     : Most independent

   • first_icu_stay = TRUE

     : Independent within each hospitalization
   • All stays: May have correlated outcomes

3. Age Shifting: Ages > 89 are shifted to protect privacy. Anchor age is 91, actual age is approximated.

4. Mortality Flags:

   • hospital_expire_flag

     : Died during this hospitalization

   • dod

     : Date of death (may be after discharge)

5. LOS Calculations:

   • los_hospital

     : Days from admission to discharge

   • los_icu

     : Days from ICU admission to ICU discharge

Example: Standard Adult Cohort

SELECT
    stay_id,
    subject_id,
    hadm_id,
    admission_age,
    gender,
    los_icu,
    hospital_expire_flag
FROM mimiciv_derived.icustay_detail
WHERE first_icu_stay = TRUE
    AND admission_age >= 18
    AND admission_age < 90  -- Exclude age-shifted patients
    AND los_icu >= 1;

Example: Readmission Analysis

-- Patients with multiple ICU stays in same hospitalization
SELECT
    hadm_id,
    COUNT(*) AS n_icu_stays
FROM mimiciv_derived.icustay_detail
GROUP BY hadm_id
HAVING COUNT(*) > 1;

-- Compare first vs subsequent stays
SELECT
    first_icu_stay,
    COUNT(*) AS n_stays,
    ROUND(AVG(los_icu), 1) AS avg_los,
    ROUND(AVG(hospital_expire_flag), 3) AS mortality
FROM mimiciv_derived.icustay_detail
GROUP BY first_icu_stay;

Example: Age Distribution

SELECT
    CASE
        WHEN admission_age < 40 THEN '<40'
        WHEN admission_age < 60 THEN '40-59'
        WHEN admission_age < 80 THEN '60-79'
        WHEN admission_age < 90 THEN '80-89'
        ELSE '90+'
    END AS age_group,
    COUNT(*) AS n_patients
FROM mimiciv_derived.icustay_detail
WHERE first_icu_stay = TRUE
GROUP BY 1
ORDER BY 1;

Related Exclusion Criteria

Common additional exclusions for research cohorts:

• Comfort care / DNR orders (see

  mimiciv_derived.code_status

  )
• Organ donors
• Very short stays (< 4-24 hours)
• Missing key data elements
• Specific diagnoses (trauma, cardiac surgery, etc.)

References

• Johnson AEW et al. "MIMIC-IV, a freely accessible electronic health record dataset." Scientific Data. 2023.