A method, performed by a control device, for deriving an engine-out NOx concentration of an exhaust gas stream in an exhaust system configured to treat exhaust gas from a combustion engine, the exhaust system comprising: a selective catalytic reduction catalyst, a dosing device adapted for injecting a reductant into the exhaust gas stream, a first NOx sensor arranged downstream of the dosing device but upstream of the selective catalytic reduction catalyst, and a second NOx sensor arranged downstream of the selective catalytic reduction catalyst; the method comprising a step of: deriving engine-out NOx concentration based on a first concentration reading obtained from the first NOx sensor and a second concentration reading obtained from the second NOx sensor, said first concentration reading being obtained while the dosing device is in operation so as to introduce reductant into the exhaust gas stream; characterized in that a turbocharger is arranged upstream of the selective catalytic reduction catalyst, and wherein the dosing device is adapted to inject the reductant towards the turbocharger, or in the turbocharger, such that the reductant is dispersed in the exhaust gas stream by usage of a rotational motion of a turbine of the turbocharger, and wherein the first concentration reading is obtained from the first NOx sensor at a first point in time, and the second concentration reading is obtained from the second NOx sensor (19 at a second point in time, and wherein the second point in time is after a time, corresponding to an estimated time for the exhaust gas stream to travel from the first NOx sensor to the second NOx sensor, has elapsed since the first point in time. the exhaust system comprising: a selective catalytic reduction catalyst, a dosing device adapted for injecting a reductant into the exhaust gas stream, a first NOx sensor arranged downstream of the dosing device but upstream of the selective catalytic reduction catalyst, and a second NOx sensor arranged downstream of the selective catalytic reduction catalyst; a selective catalytic reduction catalyst, a dosing device adapted for injecting a reductant into the exhaust gas stream, a first NOx sensor arranged downstream of the dosing device but upstream of the selective catalytic reduction catalyst, and a second NOx sensor arranged downstream of the selective catalytic reduction catalyst; the method comprising a step of: deriving engine-out NOx concentration based on a first concentration reading obtained from the first NOx sensor and a second concentration reading obtained from the second NOx sensor, said first concentration reading being obtained while the dosing device is in operation so as to introduce reductant into the exhaust gas stream; characterized in that a turbocharger is arranged upstream of the selective catalytic reduction catalyst, and deriving engine-out NOx concentration based on a first concentration reading obtained from the first NOx sensor and a second concentration reading obtained from the second NOx sensor, said first concentration reading being obtained while the dosing device is in operation so as to introduce reductant into the exhaust gas stream; characterized in that a turbocharger is arranged upstream of the selective catalytic reduction catalyst, and wherein the dosing device is adapted to inject the reductant towards the turbocharger, or in the turbocharger, such that the reductant is dispersed in the exhaust gas stream by usage of a rotational motion of a turbine of the turbocharger, and wherein the first concentration reading is obtained from the first NOx sensor at a first point in time, and the second concentration reading is obtained from the second NOx sensor (19 at a second point in time, and wherein the second point in time is after a time, corresponding to an estimated time for the exhaust gas stream to travel from the first NOx sensor to the second NOx sensor, has elapsed since the first point in time.