All no-DNA MLPA reactions will show four Q-fragments at 64, 70, 76, and 82 nt. Amplification of the Q-fragments is not dependent on sample DNA or ligation efficiency. The presence of Q-fragments in a no-DNA reaction indicates that the PCR reaction was carried out properly.
You may also see some additional peaks in your no-DNA reactions as MLPA no-DNA control reactions are more prone to non-specific peak formation than reactions that contain DNA. These non-specific peaks should not influence MLPA results when sufficient sample DNA is used, as they should be outcompeted by the amplification products of MLPA probes. At MRC-Holland we only approve probemixes that show non-specific peaks in the no-DNA reaction if they are below 25% of the median signal of the Q-fragments.
Detailed information can be found in the quality control section of the (MS-)MLPA General Protocol.
If you see a large number of peaks resembling a full peak pattern in your no-DNA control reaction, this is an indication of contamination.