According to Ruth Lawrence, Breastfeeding: a Guide for the Medical
Profession, third edition, page 477:

"Special cautions while performing this simple test should include the
following:
Use a representative, well-mixed sample.
Complete a sample of pumping from at least one breast: do not take just
a spot sample.
A well-mixed 24-hour sample can be used.
Use tube at least three-fourths filled; seal one end.
Centrifuge for 15 minutes in standard table-top centrifuge.
Occasional small layer of liquid fat on top (free fatty acids) should
not be included.
Keep tube vertical.
Make measurement within 1 hour of centrifugation."

Barb Berges BS, RN, IBCLC
Rochester, New York