Table 2. Clinicopathological variables and serum HER2 levels at baseline and after 2 cycles of treatment in the study cohort.

	Baseline sHER2 levels (N=309)							sHER2 level after 2 cycles of treatment (N=208)
	Low			High		P value		Low			High		P value
	N	%		N	%			N	%		N	%
Total	145	46.93		164	53.07			177	85.10		31	14.90
Age group						0.252							0.321
<50 years	74	23.95		73	23.62			80	38.46		17	8.17
≥50 years	71	22.98		91	29.45			97	46.63		14	6.73
Menstrual status						0.23							0.102
Premenopausal	78	25.24		77	24.92			86	41.35		20	9.62
Postmenopausal	67	21.68		87	28.16			91	43.75		11	5.29
cT						0.006							0.209
T1	6	1.94		6	1.94			5	2.40		0	0.00
T2	63	20.39		45	14.56			64	30.77		7	3.37
T3	55	17.80		67	21.68			70	33.65		13	6.25
T4	21	6.80		46	14.89			38	18.27		11	5.29
cN						0.053							0.172
N0	13	4.21		8	2.59			6	2.88		2	0.96
N1	81	26.21		74	23.95			95	45.67		11	5.29
N2	33	10.68		55	17.80			52	25.00		10	4.81
N3	18	5.83		27	8.74			24	11.54		8	3.85
Clinical stage						0.002							0.943
II	50	16.18		31	10.03			41	19.71		7	3.37
III	95	30.74		133	43.04			136	65.38		24	11.54
ER status						0.029							0.424
Negative	66	21.36		95	30.74			89	42.79		18	8.65
Positive	79	25.57		69	22.33			88	42.31		13	6.25
PgR status						0.282							0.969
Negative	98	31.72		120	38.83			125	60.10		22	10.58
Positive	47	15.21		44	14.24			52	25.00		9	4.33
Subtype						0.029							0.424
Luminal B	79	25.57		69	22.33			88	42.31		13	6.25
HER2-enriched	66	21.36		95	30.74			89	42.79		18	8.65
Overall tumor response						0.784							0.626
CR	12	3.88		12	3.88			15	7.21		2	0.96
PR	131	42.39		148	47.90			160	76.92		28	13.46
SD	2	0.65		3	0.97			2	0.96		1	0.48
PD	0	0.00		1	0.32			0	0.00		0	0.00
pCR						0.159							0.038
No	76	24.60		99	32.04			96	46.15		23	11.06
Yes	69	22.33		65	21.04			81	38.94		8	3.85
Miller-Payne grade						0.024							0.013
1	8	2.59		5	1.62			6	2.88		0	0.00
2	0	0.00		12	3.88			5	2.40		4	1.92
3	23	7.44		30	9.71			29	13.94		10	4.81
4	28	9.06		32	10.36			36	17.31		7	3.37
5	71	22.98		68	22.01			83	39.90		7	3.37
NA	15	4.85		17	5.50			18	8.65		3	1.44
iDFS events						0.015							0.003
No	138	44.66		143	46.28			166	79.81		24	11.54
Yes	7	2.27		21	6.80			11	5.29		7	3.37
Surgery						0.003							0.080
Mastectomy	117	37.86		151	48.87			151	72.60		30	14.42
Breast conserving	28	9.06		13	4.21			26	12.50		1	0.48

According to AJCC Cancer Staging Manual, Eighth Edition (2017): cT, clinical primary tumor size; cN, clinical regional lymph nodes. sHER2, serum human epidermal growth factor receptor 2; ER, estrogen receptor; PgR, progesterone receptor; HER2, human epidermal growth factor receptor 2; CR, complete response; PR, partial response; SD, stable disease; PD, progressive disease; pCR, pathological complete response; NA, not available; iDFS, invasive disease-free survival.