When an OTDR is used to measure the attenuation of multiple joined fiber lengths, the output trace can incorrectly show a joint as having gain, instead of loss. The reason for this is that adjacent fibers may have different backscatter coefficients, so the second fiber reflects more light than the first fiber fusion splicer, with the same amount of light travelling through it. If the OTDR is placed at the other end of this same fiber pair, it will measure an abnormally high loss at that joint. However if the two signals are then combined, the correct loss will be obtained. For this reason, it is common OTDR from fusion splicer manufacturer practice to measure and combine the loss from both ends of a link, so that the loss of cable joints, and end to end loss, can be more accurately measured.

The theoretical distance measuring accuracy of an OTDR is extremely good, since it is based on software and a crystal clock with an inherent accuracy of better than 0.01%. This aspect does not need subsequent calibration since practical cable length measuring accuracy is typically limited to about 1% due to: The cable length is not the same as the fiber length, the speed of light in the fiber is known with limited accuracy (the refractive index is only specified to 3 significant figures such as eg 1.45 etc), and cable length markers have limited accuracy (0.5% - 1%).