A method of determining the level of DNA-containing, biological cells of a biological source, said biological source is a raw milk sample, the method comprising the steps of: a) providing a microfluidic device comprising: - a sample reception structure, - a micro conduit system comprising a DNA gelling system comprising a DNA gelling agent, said DNA gelling system is present in dry form, b) adding a liquid sample of the biological source to the sample reception structure, c) contacting the DNA gelling system with a first portion of the sample, thereby generating DNA gel, and d) measuring a parameter correlated with the viscosity of the first portion of the sample during and/or after contacting the first portion with the DNA gelling system, e) optionally, measuring a parameter correlated with the viscosity of a portion of the sample which has not been in contact with the DNA gelling system, and f) determining the level of DNA-containing, biological cells of the biological source using the measurements of step d) and optionally also e), and wherein - the method includes step e) and uses the measured parameter of step e) in the determination of step f) and/or - the measured parameter of step d) at least partially compensates for variation in the viscosity of the liquid sample, wherein the parameter correlated with the viscosity of the first portion of the sample is one of the following: i) the flow rate of the first portion of the sample during and/or after contact with the DNA gelling system, ii) the steady-state position of a liquid in the microfluidic device, which liquid is or has been in liquid contact with the first portion of the sample during and/or after contact with the DNA gelling system, iii) presence or non-presence of liquid at a specific location in the microfluidic device, which liquid is or has been in liquid contact with the first portion of the sample during and/or after contact with the DNA gelling system, and iv) an optical property of the first portion. a) providing a microfluidic device comprising: - a sample reception structure, - a micro conduit system comprising a DNA gelling system comprising a DNA gelling agent, said DNA gelling system is present in dry form, - a sample reception structure, - a micro conduit system comprising a DNA gelling system comprising a DNA gelling agent, said DNA gelling system is present in dry form, b) adding a liquid sample of the biological source to the sample reception structure, c) contacting the DNA gelling system with a first portion of the sample, thereby generating DNA gel, and d) measuring a parameter correlated with the viscosity of the first portion of the sample during and/or after contacting the first portion with the DNA gelling system, e) optionally, measuring a parameter correlated with the viscosity of a portion of the sample which has not been in contact with the DNA gelling system, and f) determining the level of DNA-containing, biological cells of the biological source using the measurements of step d) and optionally also e), and - the method includes step e) and uses the measured parameter of step e) in the determination of step f) and/or - the measured parameter of step d) at least partially compensates for variation in the viscosity of the liquid sample, i) the flow rate of the first portion of the sample during and/or after contact with the DNA gelling system, ii) the steady-state position of a liquid in the microfluidic device, which liquid is or has been in liquid contact with the first portion of the sample during and/or after contact with the DNA gelling system, iii) presence or non-presence of liquid at a specific location in the microfluidic device, which liquid is or has been in liquid contact with the first portion of the sample during and/or after contact with the DNA gelling system, and iv) an optical property of the first portion.