Siman 164
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א נוֹטֵל אָדָם יָדָיו שַׁחֲרִית וּמַתְנֶה עֲלֵיהֶם כָּל הַיּוֹם, אֲפִלּוּ שֶׁלֹּא בִּשְׁעַת הַדַּחַק, וּבִלְבַד שֶׁלֹּא יַסִיחַ דַּעְתּוֹ מֵהֶם וְלֹא יְטַנְּפֵם (טוּר); וְאִם מַיִם מְצוּיִים לוֹ, טוֹב שֶׁיַּחֲזֹר וְיִטֹּל יָדָיו, אֲבָל לֹא יְבָרֵךְ. הַגָּה: וְהָא דִּמְהָנֵי תְּנַאי, דַּוְקָא בִּנְטִילָה שֶׁאֵינָהּ צֹרֶךְ אֲכִילָה, דּוּמְיָא דִּנְטִילַת שַׁחֲרִית, אֲבָל אִם נוֹטֵל לְצֹרֶךְ אֲכִילָה לֹא מְהָנֵי תְּנַאי בְּאוֹתָהּ נְטִילָה (בֵּית יוֹסֵף בְּשֵׁם הר"י).
1 That one may make a condition regarding handwashing. 2 sections: A person may wash his hands in the morning and stipulate that the washing covers him for the entire day, even not in a time of pressing need, provided he does not have a lapse of attention regarding them (and does not soil them) (Tur). If water is readily available it is good to wash again, but he should not recite a blessing. Rema: The stipulation is only effective for washing that is not needed for eating, similar to the morning handwashing; but if one is washing for the purpose of eating, a stipulation does not help for that washing. (Beit Yosef in the name of Ri)
ב מִי שֶׁעוֹמֵד בִּסְעֻדָּה וְנִזְכַּר שֶׁנָּגַע בְּשׁוֹק וְיָרֵךְ וּמְקוֹמוֹת הַמְכֻסִים בָּאָדָם, אוֹ שֶׁחִכֵּךְ בְּרֹאשׁוֹ וְכָל כַּיּוֹצֵא בָּזֶה, וּבִמְקוֹמוֹת הַמְטֻנָּפִים שֶׁיֵּשׁ בָּהֶם מִלְמוּלֵי זֵעָה צָרִיךְ לַחֲזֹר וְלִטֹּל יָדָיו, וִיבָרֵךְ: עַל נְטִילַת יָדָיִם.
2 One who is in the middle of a meal and remembers that he touched his thigh, upper leg, or the covered parts of the body, or that he scratched his head and similar things (and soiled areas that have accumulated perspiration residue) he must wash his hands again and recite the blessing over handwashing.