Calculate z-scores for anthropometric measures according to GIGS guidance

These functions calculate z-scores for weight-for-age (WAZs), length/height-for-age (LHAZs), weight-for-length/height (WLZs), or head circumference-for-age (HCAZs).

The growth standard used for each observation differs based on the gestational and post-menstrual age of the infant being analysed. The different procedures are based on the advised use of each growth standard, where a measurement is considered a 'birth measurement' if it is the first measurement taken from an infant and was taken <3 days after birth:

• gest_days < 24 weeks (168 days):

  • Birth: No standard available

  • Postnatal: IG-21^st Postnatal Growth standards from 27 to 64 weeks' PMA

  • Postnatal: Uncorrected WHO standard after 64 weeks' PMA

• gest_days >= 24 weeks (168 days):

  • Birth: IG-21^st Newborn Size standards at birth (including Very Preterm Newborn Size standards if born before 33 weeks).

  • Postnatal: IG-21^st Postnatal Growth standards from 27 to 64 weeks' PMA

  • Postnatal: Uncorrected WHO Child Growth standards after 64 weeks' PMA

• gest_days >= 37 weeks (259 days):

  • Birth: IG-21^st Newborn Size standards

  • Postnatal: Uncorrected WHO Child Growth standards

• gest_days >= 43 weeks (301 days):

  • Birth: No standard available

  • Postnatal: Uncorrected WHO Child Growth standards

For gigs_wlz(), two WHO standards are available:

• The weight-for-length standard, which is applied when age_days < 731.

• The weight-for-height standard, which is applied when age_days >= 731.

Usage

gigs_waz(weight_kg, age_days, gest_days, sex, id = NULL)

gigs_lhaz(lenht_cm, age_days, gest_days, sex, id = NULL)

gigs_wlz(weight_kg, lenht_cm, age_days, gest_days, sex, id = NULL)

gigs_hcaz(headcirc_cm, age_days, gest_days, sex, id = NULL)

Arguments

weight_kg

A numeric vector of length one or more with weight in kg.

age_days

A numeric vector of length one or more with ages in days.

gest_days

A numeric vector of length one or more with gestational ages in days.

sex

Character vector of length one or more with sex(es), either "M" (male) or "F" (female). This argument is case-sensitive. By default, gigs will replace elements of sex which are not "M" or "F" with NA and warn you. You can customise this behaviour using the GIGS package-level options.

id

A factor of length one or more with an ID for each observation, either ordered or unordered, containing no missing (NA) values. When supplied, id is used to ensure that only the earliest measurement for each individual is used as a birth measure. Leave this argument as NULL if all your data comes from the same individual. Default = NULL.

lenht_cm

A numeric vector of length one or more with length or height in cm. Should be recumbent length when age_days < 731, and standing height when age_days >= 731.

headcirc_cm

Numeric vector of length one or more with head circumference in cm.

Value

A numeric vector of z-scores, derived using the appropriate growth standard for each element-wise combination of gest_days and age_days. For gigs_wlz(), all birth WLZs will be missing (NA), as there is no INTERGROWTH-21^st Newborn standard for weight-for-length.

Note

These functions expect vectors which are recyclable with vctrs::vec_recycle_common().

Examples

gest_days <- c(rep(35 * 7, 3), rep(35 * 7, 3))
age_days <- c(0, 100, 500, 2, 100, 500)
sex <- rep.int(c("M", "F"), c(3, 3))
wt_kg <- c(3, 6, 9, 3, 6, 9)
len_cm <- rep.int(c(52.2, 60.4, 75), 2)
head_cm <- rep.int(c(30, 40, 49), 2)
ids <- factor(rep.int(c("A", "B"), c(3, 3)))

# Weight-for-age z-score (WAZ)
waz <- gigs_waz(weight_kg = wt_kg,
                age_days = age_days,
                gest_days = gest_days,
                sex = sex,
                id = ids)
#> Warning: There was 1 'at birth' observation where `age_days` > 0.5.
#> ℹ This occurred for ID B.
print(waz)
#> [1]  1.2230003  0.7413417 -1.4866806  1.5420839  1.4395342 -0.7859202

# Note - if you don't specify 'id' you'll get different results!
waz_no_id <- gigs_waz(weight_kg = wt_kg,
                      age_days = age_days,
                      gest_days = gest_days,
                      sex = sex)
print(waz == waz_no_id)
#> [1]  TRUE  TRUE  TRUE FALSE  TRUE  TRUE

# Length/height-for-age z-score (LHAZ)
lhaz <- gigs_lhaz(lenht_cm = len_cm,
                  age_days = age_days,
                  gest_days = gest_days,
                  sex = sex,
                  id = ids)
#> Warning: There was 1 'at birth' observation where `age_days` > 0.5.
#> ℹ This occurred for ID B.
print(lhaz)
#> [1]  2.8236856  0.8876444 -2.1690082  3.4154524  1.8205012 -1.4423063

# Weight-for-length/height z-score (WLZ)
#   Note: There's no at-birth standards for weight-for-length, so elements 1
#   and 4 have `NA` values
wlz <- gigs_wlz(weight_kg = wt_kg,
                lenht_cm = len_cm,
                age_days = age_days,
                gest_days = gest_days,
                sex = sex,
                id = ids)
#> Warning: There was 1 'at birth' observation where `age_days` > 0.5.
#> ℹ This occurred for ID B.
print(wlz)
#> [1]         NA  1.6408675 -0.6621871         NA  1.3314150 -0.1829182

# Head circumference-for-age z-score (HCAZ)
hcaz <- gigs_hcaz(headcirc_cm = head_cm,
                  age_days = age_days,
                  gest_days = gest_days,
                  sex = sex,
                  id = ids)
#> Warning: There was 1 'at birth' observation where `age_days` > 0.5.
#> ℹ This occurred for ID B.
print(hcaz)
#> [1] -1.5588136  0.9510032  1.4502711 -1.3289704  1.7433668  2.2148421