Table 1: Small parameters and critical sets for the standard, reverse, and total QSSA.

Each of the critical manifolds that appear when either η, $\underset{¯}{\epsilon }$ or ν are identically zero correspond to zero product formation. Hence, the tQSSA is valid in all three cases. The notation ${\overline{c}}_0(\overline{p};K_S)$ denotes the c-nullcline with K_M replaced with K_S, which occurs when k₂ = 0.

Parameter	Approximation	Critical Manifold (Set)
η	sQSSA	$1-\overline{s}-\overline{p}=0$
$\underset{¯}{\epsilon }$	rQSSA	$\{(\widehat{c},\overline{p})\in {\mathbb{R}}^{+}\mid (1-\widehat{c})(1-\widehat{c}-\overline{p})=0\}$
ν	tQSSA	${\overline{c}}_0(\overline{p};K_S)$