What is 21 in rate-westergren test?

That depends on your age and sex

see below for more details on this.

Note: mm/hr. = millimeters per hour.(sourced from wikipedia)

Westergren's original normal values (men 3mm and women 7mm)[5] made no allowance for a person's age and in 1967 it was confirmed that ESR values tend to rise with age and to be generally higher in women.[6] Values are increased in states of anemia,[7] and in black populations.[8]

Adults

The widely used[9] rule for calculating normal maximum ESR values in adults (98% confidence limit) is given by a formula devised in 1983:

{\rm ESR}\ (mm/hr) \le \frac {{\rm Age}\ ({\it in\ years}) + 10\ ({\it if\ female})}{2}

ESR reference ranges from a large 1996 study with weaker confidence limits:

(ESR 95% limits) Age (years)

Men-----20 years old 10, 55 years old 14, 90 yearolds 19.

Women-20 years old 15, 55 years old 21, 90 yearolds 23.

Children

Normal values of ESR have been quoted as 1[12] to 2[13] mm/hr at birth, rising to 4 mm/hr 8 days after delivery,[13] and then to 17 mm/hr by day 14.

Typical normal ranges quoted are:

Newborn: 0 to 2 mm/hr

Neonatal to puberty: 3 to 13 mm/hr, but other laboratories place an upper limit of 20. sourced from wikipedia