This application describes a method comprising: (a) providing a reference standard or a planar section thereof, the reference standard comprising (i) a support medium; and (ii) a quantity of a detectable entity supported by the support medium; in which the detectable entity has an elongate path; in which a predetermined amount of the detectable entity is present in a defined region in a cross section of the referenced standard; (b) obtaining a first reference signal indicative of the presence or quantity of detectable entity in the reference standard, planar section thereof, or the defined region; (c) providing a biological sample and obtaining a second signal indicative of the presence or quantity of detectable entity in the biological sample, or a component thereof; and (d) comparing the reference signal obtained in (b) against the second signal obtained in (c). Preferably, the method is used to indicate the presence, quantity or concentration of a detectable entity in a biological sample.