Study	Tests included	Tumour type	Grade	+++	++‐	+‐+	‐++	+‐‐	‐‐+	‐+‐	‐‐‐	Total with test results	Notes
Belaud‐Rotureau 2006	A: FISH (1p36.3 (D1Z2)/1q12 (D1Z1) and 19q13.3/19pter probes and manual analysis) B: FISH (1p36/1q25 and 19q13/19p13 Abbott Vysis probe set and manual analysis) C: FISH (1p36/1q25 and 19q13/19p13 Abbott Vysis probe set and automated analysis)	Oligodendroglioma	III	1	0	0	0	0	0	0	0	1	—
		Oligoastrocytoma	II	1	0	0	0	0	0	0	1	2
		Oligoastrocytoma	III	1	0	0	0	0	0	0	6	7
Blesa 2009 (aCGH vs FISH vs PCR)	A: aCGH B: FISH C: PCR	Anaplastic oligoastrocytoma	III	0	0	0	0	0	0	0	1	1	Some differences between Table 2 in text and the Supplementary Table. Since the Supplementary Table had more information, we extracted from this. Relative deletions on FISH were classified as a negative result as they were not absolute deletions.
		Oligodendroglioma	II	1	1	0	0	0	0	0	0	2
Blesa 2009 (FISH vs MLPA vs PCR)	A: FISH B: MLPA C: PCR	Anaplastic oligodendroglioma	III	1	0	0	0	0	0	0	1	2	Some differences between Table 2 in text and the Supplementary Table. Since the Supplementary Table had more information, we extracted from this. Relative deletions on FISH were classified as a negative result as they were not absolute deletions.
		Oligodendroglioma	II	1	0	0	3	0	0	0	2	6
		Anaplastic oligoastrocytoma	III	0	0	0	0	0	0	0	1	1
Blesa 2009 (aCGH vs MLPA vs PCR)	A: aCGH B: MLPA C: PCR	Oligodendroglioma	II	1	0	0	0	0	0	0	1	2	Some differences between Table 2 in text and the Supplementary Table. Since the Supplementary Table had more information, we extracted from this. Relative deletions on FISH were classified as a negative result as they were not absolute deletions.
Burger 2001	A: CGH B: FISH C: PCR	Oligodendroglioma	II	5	0	0	0	0	0	0	0	5	We used the "review" diagnosis (still done without knowledge of the results of the tests). Overall results for PCR‐based LOH for 1p and 19q not given. We assumed that if results were homozygous or indeterminant at all loci examined that codeletion was present. We excluded the second sample for participant T276. We removed participant T272 because they were included in Hatanpaa 2003b.
		Malignant oligodendroglioma	III	1	0	0	0	0	0	0	0	1
		Astrocytoma	II	0	0	0	0	0	0	0	2	2
		Astrocytoma	III	0	0	0	0	0	0	0	6	6
		Malignant mixed glioma	III	0	0	0	0	0	0	0	1	1
		Glioblastoma	IV	0	0	0	0	0	0	0	2	2
Dahlback 2011	A: CGH B: G‐banding C: PCR	Oligoastrocytoma	II	0	0	4	0	0	3	0	6	13	We excluded people with failure results. We could not make the numbers that we derived from Table 1 correspond to what was reported in the text.
		Oligodendroglioma	II	0	0	4	0	0	0	0	1	5
Hatanpaa 2003a (assay development and non‐blinded validation cohort) (FISH vs PCR with comparison to normal DNA vs PCR without comparison to normal DNA)	A: FISH B: PCR (comparison to normal DNA) C: PCR (microsatellite)	Astrocytoma	II	0	0	0	0	0	0	0	1	1	Participants from Smith 1999. We removed D1S534 from the regions analysed by PCR with comparison to normal DNA as this was not used to assess 1p/19q status. We used the histological diagnosis from this paper (rather than from Smith 1999).
		Oligodendroglioma	II	1	0	0	0	0	0	0	0	1
		Oligodendroglioma	III	2	0	0	0	0	0	0	0	2
Hatanpaa 2003b (blinded validation cohort) (FISH vs PCR with comparison to normal DNA vs PCR without comparison to normal DNA)	A: FISH B: PCR (comparison to normal DNA) C: PCR (microsatellite)	Astrocytoma	IV	0	0	0	0	0	0	0	1	1	Participants from Smith 1999. We removed D1S534 from the regions analysed by PCR with comparison to normal DNA as this was not used to assess 1p/19q status. We used the histological diagnosis from this paper (rather than from Smith 1999).
		Oligodendroglioma	II	1	0	0	0	0	0	0	0	1
Horbinski 2012	A: FISH (cut‐off < 0.87) B: FISH (cut‐off < 0.75) C: PCR	Oligodendroglioma	II, III	61	15	3	0	5	1	0	26	111	Discrepancy between text (and what they calculated in Table 2) and Table 1 – we used text and Table 2.
Mohapatra 2006a	A: aCGH B: FISH C: PCR	"Oligodendroglial tumours"	II, III	16	0	1	0	1	0	0	10	28	32 tumours included in the study, had results for 28 and results not broken down by tumour subtype, apart for the discordant cases.
Pesenti 2017 (aCGH vs MS vs PCR)	A: aCGH B: MS C: PCR	Oligodendroglioma	II	1	0	0	0	0	0	0	0	1	—
		Anaplastic oligodendroglioma	III	1	0	0	0	0	0	0	0	1
		Glioblastoma	IV	0	0	0	0	0	0	0	2	2
Pesenti 2017 (FISH vs MS vs PCR)	A: FISH B: MS C: PCR	Oligodendroglioma	II	3	0	0	0	0	0	0	0	3	—
		Anaplastic oligodendroglioma	III	2	0	0	0	0	0	0	0	2
		Astrocytoma	II	0	0	0	0	0	0	0	1	1
		Glioblastoma	IV	0	0	0	0	0	0	0	4	4
Smith 1999	A: CGH B: FISH C: PCR	Astrocytoma	III	0	0	0	0	0	0	0	3	3	Quote: "LOH, FISH, and CGH were performed as previously described (Cliby et al., 1993; Ritland et al., 1995; Qian et al., 1995; Mohapatra et al., 1995, 1998; Piper et al., 1995)". Overall results for 1p and 19q for PCR not given. We assumed that if results were confirmed allelic loss/presumed allelic loss/homozygous at all loci on 1p except the most centromeric (D1S534‐ removed from list of loci examined) and all loci on 19q examined that codeletion was present. Overall results for FISH also not reported. We defined codeletion as hemizygous deletion of 1p36, 1q13.1‐q13.2 and 19q13.3.
		Astrocytoma	IV	0	0	0	1	0	0	0	17	18
		Mixed	II	2	0	0	0	0	0	0	0	2
		Mixed	III	0	1	0	0	0	0	0	2	3
		Mixed	IV	0	0	0	1	0	0	1	1	3
		Oligodendroglioma	II	4	0	0	0	0	0	1	4	9
		Oligodendroglioma	III	5	0	0	0	0	0	0	1	6
		Oligodendroglioma	IV	0	0	0	0	1	0	0	0	1