APPENDIX 5 Cervical mucus 249
Spinnbarkeit is scored as follows: 0 = <1 cm 1 = 1–4 cm 2 = 5–8 cm 3 = 9 cm or more A5.3.5 Cellularity It is recommended that all cell counts be expressed in cells per Pl. An estimate of the number of leukocytes and other cells in the cervical mucus is traditionally based on the number counted per high-power microscope field (HPF) (see Box A5.2). Box A5.2 Volume observed per high-power field in a 100-Pm-deep mucus preparation The volume of mucus observed in each microscope field depends on the area of the field (Sr2, where S is approximately 3.142 and r is the radius of the microscopic field) and the depth of the chamber (here 100 Pm). The diameter of the microscope field can be measured with a stage micrometer or can be estimated by dividing the diameter of the aperture of the ocular lens by the magnification of the objective lens. With a ×40 objective and a ×10 ocular of aperture 20 mm, the microscope field has a diameter of approximately 500 Pm (20 mm / 40). In this case, r = 250 Pm, r2 = 62 500 Pm2, Sr2 = 196 375 Pm2 and the volume is 19 637 500 Pm3 or about 20 nl.
Thus, a count of 10 cells per HPF is approximately equivalent to 10 cells per 20 nl, or 500 cells per Pl. As the number of cells counted is low, the sampling error is high; a replicate count of 10 has a sampling error of 22% (see Table 2.2), so the value could lie anywhere between 280 and 720 cells per Pl. The rank scores for cells are: 0 = >20 cells per HPF or >1000 cells per Pl 1 = 11–20 cells per HPF or 501–1000 cells per Pl 2 = 1–10 cells per HPF or 1–500 cells per Pl 3 = 0 cells A5.3.6 pH The pH of cervical mucus from the endocervical canal should be measured with pH paper, range 6.0–10.0, in situ or immediately following collection. If the pH is measured in situ, care should be taken to avoid touching the exocervical mucus, which always has a pH lower (more acidic) than that of mucus in the endocervical canal. Care should also be taken to avoid contamination with secretions of the vagina, which have a low pH. Spermatozoa are susceptible to changes in pH of the cervical mucus. Acid mucus immobilizes spermatozoa, whereas alkaline mucus may enhance motility. Exces-