1. Primer Length
Multiplex PCR assays involve designing of large number of primers, hence it is required that the designed primer should be of appropriate length. Usually, primers of short length, in the range of 18-22 bases are used.
2. Melting Temperature
Primers with similar Tm, preferably between 55°C-60°C are used. For sequences with high GC content, primers with a higher Tm (preferably 75°C-80°C) are recommended. A Tm variation of between 3°-5° C is acceptable for primers used in a pool.
3. Specificity
It is important to consider the specificity of designed primers to the target sequences, while preparing a multiplex assay, especially since competition exists when multiple target sequences are in a single reaction vessel.
4. Avoid Primer Dimer Formation
The designed primers should be checked for formation of primer dimers, with all the primers present in the reaction mixture. Dimerization leads to unspecific amplification.