A device for determining a concentration of cells of a given size in a sample of fluid, the sample comprising a plurality of cells, comprising: a light source arranged to emit light along a central axis to illuminate the plurality of cells in the fluid sample; a container (120, 200, 250) to contain the sample: a detector arranged to receive light at a plurality of displacements from the central axis the light having passed through the plurality of cells in the fluid sample, and to produce a plurality of signals, each associated with, and indicative of an intensity of the received light at, a respective one of the plurality of displacements; and a processor arranged to: obtain a first light intensity reading at a first displacement from the central axis; obtain a second light intensity reading at a second displacement from the central axis; determine a difference between the first and second light intensity readings; if the difference is more than a predefined threshold, determine, using a first method, the concentration of cells of a given size on the basis of the first and second light intensity readings and the difference therebetween and, if the difference between the first and second light intensity readings is less than the predefined threshold, determine, using a second method, the concentration of cells of the given size on the basis of the first and second light intensity readings and the difference therebetween, wherein the first and second methods are different. a light source arranged to emit light along a central axis to illuminate the plurality of cells in the fluid sample; a container (120, 200, 250) to contain the sample: a detector arranged to receive light at a plurality of displacements from the central axis the light having passed through the plurality of cells in the fluid sample, and to produce a plurality of signals, each associated with, and indicative of an intensity of the received light at, a respective one of the plurality of displacements; and a processor arranged to: obtain a first light intensity reading at a first displacement from the central axis; obtain a second light intensity reading at a second displacement from the central axis; determine a difference between the first and second light intensity readings; if the difference is more than a predefined threshold, determine, using a first method, the concentration of cells of a given size on the basis of the first and second light intensity readings and the difference therebetween and, if the difference between the first and second light intensity readings is less than the predefined threshold, determine, using a second method, the concentration of cells of the given size on the basis of the first and second light intensity readings and the difference therebetween, wherein the first and second methods are different. a detector arranged to receive light at a plurality of displacements from the central axis the light having passed through the plurality of cells in the fluid sample, and to produce a plurality of signals, each associated with, and indicative of an intensity of the received light at, a respective one of the plurality of displacements; and a processor arranged to: obtain a first light intensity reading at a first displacement from the central axis; obtain a second light intensity reading at a second displacement from the central axis; determine a difference between the first and second light intensity readings; if the difference is more than a predefined threshold, determine, using a first method, the concentration of cells of a given size on the basis of the first and second light intensity readings and the difference therebetween and, if the difference between the first and second light intensity readings is less than the predefined threshold, determine, using a second method, the concentration of cells of the given size on the basis of the first and second light intensity readings and the difference therebetween, wherein the first and second methods are different. obtain a first light intensity reading at a first displacement from the central axis; obtain a second light intensity reading at a second displacement from the central axis; determine a difference between the first and second light intensity readings; if the difference is more than a predefined threshold, determine, using a first method, the concentration of cells of a given size on the basis of the first and second light intensity readings and the difference therebetween and, if the difference between the first and second light intensity readings is less than the predefined threshold, determine, using a second method, the concentration of cells of the given size on the basis of the first and second light intensity readings and the difference therebetween, wherein the first and second methods are different.