Table 1.

Application of the Harris and Boyd partition criteria for pregnancy age

Pregnancy age	Reference interval (×10^9/L)	z score
0 week	3.7–9.3
4 weeks	4.7–11.9	8.27*
7 weeks	5.7–14.4	6.20*
40 weeks	6.0–15.0	1.41

Z score was calculated by the formular [(‾x₂ -‾x₁)/(s₁²/n₁ + s₂²/n₂)]^1/2, x₂ and x₁ are the adjacent points in 0, 4, 7, 40 weeks of gestation. When z score > = 5[(n₁ + n₂)/240]^1/2, a reference interval partitioning was required. *Reference interval partitioning was necessary according to the z score from the Harris and Boyd’s test