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CModule::IncludeModule("iblock");

$elements = \Bitrix\Iblock\Elements\ElementZapisTable::getList([
    'order'  => ['ISINT.VALUE'=>'DESC'],
    'select' => ['ID', 'NAME', 'ISINTVAL' => 'ISINT.VALUE'],
    'filter' => ['=ACTIVE' => 'Y'],
    'limit'  => 1,
])->fetchAll();
foreach ($elements as $element) {
   echo "<pre>"; var_dump($element["ISINTVAL"]); echo "</pre>";//max
}
$elements = \Bitrix\Iblock\Elements\ElementZapisTable::getList([
    'order'  => ['ISINT.VALUE'=>'ASC'],
    'select' => ['ID', 'NAME', 'ISINTVAL' => 'ISINT.VALUE'],
    'filter' => ['=ACTIVE' => 'Y'],
    'limit'  => 1,
])->fetchAll();
foreach ($elements as $element) {
   echo "<pre>"; var_dump($element["ISINTVAL"]); echo "</pre>";//min
}