Binding assay signal analysis

1. A system for determining a sample concentration of target entities in a sample, comprising:

an assay assembly having a plurality of assay areas connected in series such that a sample flowing through the assay assembly flows past each assay area in sequence, and wherein each assay area comprises a plurality of probe entities immobilized at a surface of the assay area, the probe entities being arranged to bind to the target entities in the sample, such that the concentration of the target entities is depleted as the sample flows from one of the assay areas to the next; and

a processor arranged to model assay data, wherein assay data comprises data points of respective local measurements indicative of a local concentration of the target entities at each of a plurality of assay areas of an assay assembly, wherein the assay areas are connected in series such that a sample flowing through the assay assembly flows past each assay area in sequence, wherein the processor:

obtains the assay data;

models the assay data with a parameterized function, wherein the parameterized function is a logistic function that is proportional to

DP

max

1

+

exp

⁡

[

Shape

⨯

(

DZ

i

+

Offset

)

]

wherein DP max and Shape are fixed parameters of the assay assembly, Offset is a parameter dependent on a sample target concentration, and DZ i is a quantity indicating the position of the respective assay area, i, in a sample flow sequence; and

determines a value related to the sample concentration based on the Offset parameter, whereby the value related to the sample concentration allows the sample concentration of target entities in the sample to be determined.

2. The system according to claim 1 , wherein the parameterized function is derived from assay data sets obtained for a range of sample concentrations.

3. The system according to claim 1 , wherein one of the one or more parameters is indicative of an offset amount which offsets the quantity indicative of the position of the assay area in the sample flow sequence such that the parameterized function is a function of the local measurements against the quantity indicative of the position of the assay area in the sequence, offset by the offset amount.

4. The system according to claim 3 , wherein the offset amount is determined by minimizing a difference between the respective local measurement and a corresponding value of the parameterized function for each assay area of the assay assembly.

5. The system according to claim 1 , wherein the parameterized function is characteristic of the assay assembly.

6. The system according to claim 5 , wherein the parameterized function is at least in part defined by one or more fixed parameters characteristic of the assay assembly.

7. The system according to claim 6 , wherein the one or more fixed parameters are determined from data sets of local measurements against the quantity obtained for respective sample target concentrations spanning a range of sample target concentrations.

8. The system according to claim 1 , wherein the processor determines the value based on at least one of the one or more parameters using a calibration function, wherein the calibration function comprises a first function for use at sample concentrations of target entities above a given value, and a second function for use at sample concentrations of target entities below the given value.

9. The system according to claim 1 , wherein the value related to the sample concentration is determined using a calibration function and wherein the calibration function comprises a first function for use at sample concentrations of target entities above a given value, and a second function for use at sample concentrations of target entities below the given value, wherein the first function is a function of Offset, and the second function is a function of:

DP

max

1

+

exp

⁡

[

Shape

⨯

Offset

]

10. The system according to claim 1 , wherein each local measurement is indicative of a variation in a refractive index at the surface of the respective assay area.

11. The system according to claim 10 , wherein the refractive index at the surface of the respective assay area is determined based on the detection of a change in Surface Plasmon Resonance.

12. The system according to claim 10 , wherein the variation in the refractive index at the surface of the respective assay area is amplified by an amplifier solution flowing past the respective assay area, wherein the amplifier solution is arranged to interact with target entities bound to the surface of the assay area such that the variation in the refractive index at the respective assay area is amplified when the amplifier has interacted with the bound target entities.

13. The system according to claim 12 , wherein each local measurement comprises a difference between a pre-amplification signal and post-amplification signal, wherein the pre-amplification signal has been detected after interaction of the sample with the respective assay area and before interaction of the amplifier solution with target entities bound to the respective assay area, and the post-amplification signal has been detected after interaction of the amplifier solution with target entities bound to the respective assay area.

14. The system according to claim 13 , wherein the processor models the assay data using an adjustment term to account for a bulk refractive index of the sample.

15. The system according to claim 14 , wherein the adjustment term is determined based on a difference between a baseline signal detected prior to interaction of the sample with the assay area and the pre-amplification signal.

16. The system according to claim 12 , wherein each local measurement indicates a rate at which the amplifier solution interacts with the respective assay area.

17. The system according to claim 12 , wherein each local measurement comprises a measurement indicative of the time taken from introduction of the amplifier solution into the respective assay area to detection of a signal feature, for example a maximum or threshold signal amplitude.

18. The system according to claim 1 , wherein an amount of probe entities with target entities between each pair of the plurality of assay areas is substantially constant.

19. The system according to claim 1 where the quantity indicating the position of the assay area in the sequence indicates is indicative of an amount of probe entities upstream of the position of the assay area.

20. The system according to claim 1 , wherein each assay area is connected to the next assay area in the sequence via a conduit.

21. The system according to claim 1 , wherein the processor obtains the assay data by:

introducing a sample into the assay assembly;

causing the sample to flow through the assay assembly; and

carrying out local measurements at each assay area.