def parse_field(field_string):
    """
    Get a tuple of the first entries found in an psimitab column
    (xref, value, desc)
    :param field_string:
    :return: A tuple (xref, value, desc) - desc is optional
    """
    parts = field_string.split(":")
    xref = parts[0]
    rest = parts[1]
    if "(" in rest:
        subfields = rest.split("(")
        value = subfields[0]
        desc = subfields[1][:-1]
    else:
        value = rest
        desc = "NA"
    return xref, value, desc