The SPDZ paper explains how the SPDZ protocol can be used to find the sum/product of secret values and find the sum/product of a secret and a non-secret value (page 6).
The paper seems to say that from this you can do any other operation. How would you use this to do secure less-than comparison?
The Bristol SPDZ implementation tutorial program (line 19) shows that their implementation has a way of securely comparing two secret values to get a secret result. How did they implement this? It doesn't seem to be in the paper.