  We present a search for transient radio sources on timescales of 2 to 9 yr at
150 MHz. This search is conducted by comparing the first Alternative Data
Release of the TIFR GMRT Sky Survey (TGSS ADR1) and the second data release of
the LOFAR Two-metre Sky Survey (LoTSS DR2). The overlapping survey area covers
5570 $\rm{deg}^2$ on the sky, or 14 per cent of the total sky. We introduce a
method to compare the source catalogues that involves a pair match of sources,
a flux density cutoff to meet the survey completeness limit and a newly
developed compactness criterion. This method is used to identify both transient
candidates in the TGSS source catalogue that have no counterpart in the LoTSS
catalogue and transient candidates in LoTSS without a counterpart in TGSS. We
find that imaging artefacts and uncertainties and variations in the flux
density scales complicate the transient search. Our method to search for
transients by comparing two different surveys, while taking into account
imaging artefacts around bright sources and misaligned flux scales between
surveys, is universally applicable to future radio transient searches. No
transient sources were identified, but we are able to place an upper limit on
the transient surface density of $<5.4 \cdot 10^{-4}\ \text{deg}^{-2}$ at 150
MHz for compact sources with an integrated flux density over 100 mJy. Here we
define a transient as a compact source with flux density greater than 100 mJy
that appears in the catalogue of one survey without a counterpart in the other
survey.
