TeX/Latex - Salto de línea en celda de tabla

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% Cabecera del documento %

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\documentclass[a4paper,oneside,openany,12pt]{book}

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% Documento %

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\begin{document}

Bla, bla, bla\ldots

\begin{table}[hbp]\begin{center}\begin{tabular}{cc}

 \textbf{A} & \textbf{B} \\

 \hline

 Bla & Ble \\

 \parbox{7em}{Hs-cTnT\\ (p99=14ng/L)} & Blu \\

 \end{tabular}

 \caption{palabrer\'ia.}

\end{center}\end{table}

Pero me gusta m\'as centrarlo y usar f\'ormulas\ldots

\begin{table}[hbp]\begin{center}\begin{tabular}{cc}

 \textbf{A} & \textbf{B} \\

 \hline

 Bla & Ble \\

 \parbox{7em}{\centering Hs-cTnT\\ $\left(p99=14\frac{ng}{L}\right)$} & Blu \\

 \end{tabular} 

 \caption{palabrer\'ia.}

\end{center}\end{table}

\end{document}