The Theory
Spike recovery is a useful tool for monitoring the accuracy of an analytical method or immunoassay. Manipulation of a sample can sometimes result in loss of an analyte leading to erroneously low measured values.

The physical composition of the sample or sample matrix may also introduce compounds that interfere with accurate measurement of the analyte. Spike recovery assists in accounting for the percent loss of an analyte and may detect interfering substances.

The Method
A specified amount of spike can be added at varying stages of the protocol. Spiking the assay system can be performed prior to sample preparation and analysis procedures. Post-sample preparation analysis requires spiking during or after dilution of the sample. The spiked analyte should always be of the same molecular form as that in the sample.

The amount of spike added to the unknown sample is determined by the investigator based on assay conditions and specific characteristics of the analyte being measured. The quantity added will be based on the normal values found in the system under investigation. The spike quantity should be as large as possible, while remaining within the calibration range of the kit. A large spike is very useful when levels of natural analyte are very low and minimizes the effect of instrument or recovery calculation variability.

The Calculation
Spike recovery is a correction factor. The percent recovery is the proportion of the added spike and the amount detected following analysis. The remaining percent is attributed to loss of matrix and/or chemical interference.

Once the percent spike recovery is calculated, a correction factor can be determined. Applying the correction factor to the recovered analyte adjusts results to reflect the loss of analyte or assay interference.