{"id":66655,"date":"2026-10-12T09:04:07","date_gmt":"2026-10-12T06:04:07","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66655"},"modified":"2026-10-12T09:04:07","modified_gmt":"2026-10-12T06:04:07","slug":"fractional-excretion-of-sodium-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/medical\/fractional-excretion-of-sodium-calculator\/","title":{"rendered":"Fractional Excretion of Sodium Calculator"},"content":{"rendered":"<section class=\"not-prose calculator-box\" aria-labelledby=\"calculator-title\"><div class=\"flex items-baseline sm:items-center justify-between gap-2 sm:gap-3\"><div class=\"flex flex-col sm:flex-row sm:items-center gap-2\"><span class=\"icon icon-calculator text-primary-700! text-base! dark:text-primary-300!\" aria-hidden=\"true\"><\/span><h2 id=\"calculator-title\" class=\"text-lg font-display font-bold\">Evaluate Prerenal vs Intrinsic Kidney Injury from Urine and Blood Values<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66655\" data-favorite-title=\"FENa\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/medical\/fractional-excretion-of-sodium-calculator\/\" data-favorite-excerpt=\"Estimate the fractional excretion of sodium (FENa) from urine and serum sodium and creatinine values, and see whether your result points to a prerenal or an intrinsic cause of acute kidney injury.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-flask\">\n    <span class=\"icon icon-shape-star-empty\" aria-hidden=\"true\"><\/span>\n  <\/button>\n  <div class=\"calcdash-favorite__tip\" aria-hidden=\"true\">\n   <span class=\"favorite-tooltip\" id=\"favorite-tooltip\"><\/span>\n  <\/div>\n  <span class=\"sr-only\" role=\"status\" id=\"favorite-status\"><\/span>\n<\/div><\/div><form action=\"https:\/\/wpcalc.com\/en\/medical\/fractional-excretion-of-sodium-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66655\" data-post=\"66655\"><fieldset class=\"fieldset-input is-2\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field has-term\" id=\"input-1\"><label for=\"field-1\">Urine sodium<\/label><div class=\"term\">U\u2099\u2090<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"urine_sodium\" id=\"field-1\" aria-describedby=\"calc-unit-1 calc-info-1\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"300\" value=\"40\"\/><div class=\"unit\" id=\"calc-unit-1\">mmol\/L<\/div><small id=\"calc-info-1\">Same numeric value in mEq\/L<\/small><\/div><div class=\"field has-term\" id=\"input-2\"><label for=\"field-2\">Serum sodium<\/label><div class=\"term\">S\u2099\u2090<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"serum_sodium\" id=\"field-2\" aria-describedby=\"calc-unit-2 calc-info-2\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"100\" max=\"180\" value=\"138\"\/><div class=\"unit\" id=\"calc-unit-2\">mmol\/L<\/div><small id=\"calc-info-2\">Same numeric value in mEq\/L<\/small><\/div><div class=\"field has-term\" id=\"input-3\"><label for=\"field-3\">Urine creatinine<\/label><div class=\"term\">U\ua700\u1d63<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"text\" name=\"urine_creatinine\" id=\"field-3\" aria-describedby=\"calc-info-3\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"2000\" value=\"60\"\/><div class=\"grid grid-cols-1\"><select id=\"field-4\" name=\"unit\" aria-label=\"Unit\"><option value=\"mgdl\" selected>mg\/dL<\/option><option value=\"umol\">\u00b5mol\/L<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-3\">From a random (spot) urine sample<\/small><\/div><div class=\"field has-term\" id=\"input-4\"><label for=\"field-5\">Serum creatinine<\/label><div class=\"term\">S\ua700\u1d63<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"text\" name=\"serum_creatinine\" id=\"field-5\" aria-describedby=\"calc-info-5\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0.1\" max=\"20\" value=\"2\"\/><div class=\"grid grid-cols-1\"><select id=\"field-6\" name=\"unit\" aria-label=\"Unit\"><option value=\"mgdl\" selected>mg\/dL<\/option><option value=\"umol\">\u00b5mol\/L<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-5\">From the same time point as the urine sample<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66655\">Calculate<\/button><button type=\"reset\">Reset<\/button><\/div><div class=\"field is-checkbox hidden!\" id=\"field-auto-calc\"><input type=\"checkbox\" id=\"auto-calc\"><label for=\"auto-calc\">Calculate automatically<\/label><small>If enabled, the result will update automatically when you change any value.<\/small><\/div><div class=\"fieldset-result is-hidden\" aria-labelledby=\"result-title\" aria-live=\"polite\" role=\"region\"> <h3 class=\"result-title bg-gradient-to-r from-primary-50 to-gray-50 dark:from-primary-900 dark:to-gray-800\" id=\"result-title\"><span class=\"icon icon-s-pulse\" aria-hidden=\"true\"><\/span> Your FENa Result<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Fractional excretion of sodium<\/span><span class=\"result-term\">FENa<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\"%\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-2\"><span class=\"field-title\"><span>Interpretation<\/span><\/span><span class=\"field-value\" id=\"result-2\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-3\"><span class=\"field-title\"><span>What this means<\/span><\/span><span class=\"field-value\" id=\"result-3\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><\/div><div class=\"result-actions\"><button type=\"button\" class=\"copy-link\"><svg viewBox=\"0 0 20 20\" fill=\"currentColor\" aria-hidden=\"true\"><path d=\"M12.232 4.232a2.5 2.5 0 0 1 3.536 3.536l-1.225 1.224a.75.75 0 0 0 1.061 1.06l1.224-1.224a4 4 0 0 0-5.656-5.656l-3 3a4 4 0 0 0 .225 5.865.75.75 0 0 0 .977-1.138 2.5 2.5 0 0 1-.142-3.667l3-3Z\"\/><path d=\"M11.603 7.963a.75.75 0 0 0-.977 1.138 2.5 2.5 0 0 1 .142 3.667l-3 3a2.5 2.5 0 0 1-3.536-3.536l1.225-1.224a.75.75 0 0 0-1.061-1.06l-1.224 1.224a4 4 0 1 0 5.656 5.656l3-3a4 4 0 0 0-.225-5.865Z\"\/><\/svg><span class=\"copy-link__text\">Copy link to this calculation<\/span><\/button><span class=\"sr-only\" role=\"status\" id=\"copy-link-status\"><\/span><\/div><\/div><a href=\"#respond\" class=\"hidden transition-opacity duration-300 opacity-0 w-50 text-sm justify-center items-center gap-2 px-4 py-2 rounded bg-gray-200 text-gray-700 hover:bg-gray-300\" id=\"leave-comment\"><span class=\"icon icon-comments\" aria-hidden=\"true\"><\/span>Leave a Comment<\/a><\/form><\/section><section id=\"calc-reactions\" class=\"not-prose hidden my-12 bg-gradient-to-r from-primary-50 to-gray-50 border border-indigo-100 rounded-xl px-6 py-4 shadow-sm\" data-post=\"66655\" aria-live=\"polite\" aria-labelledby=\"calc-reactions-title\"><p id=\"calc-reactions-title\" class=\"text-sm text-gray-500 text-center mb-2\">How did this result make you feel?<\/p><div role=\"group\" aria-labelledby=\"calc-reactions-title\" class=\"grid grid-cols-3 sm:flex sm:flex-row sm:justify-around gap-4 sm:items-center sm:flex-wrap\"><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"like\"><span class=\"reaction-count\" data-reaction-count=\"like\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude00 <\/span><span class=\"reaction-description font-medium\">Like <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"helpful\"><span class=\"reaction-count\" data-reaction-count=\"helpful\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\udca1 <\/span><span class=\"reaction-description font-medium\">Helpful <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"confused\"><span class=\"reaction-count\" data-reaction-count=\"confused\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude15 <\/span><span class=\"reaction-description font-medium\">Confused <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"disappointed\"><span class=\"reaction-count\" data-reaction-count=\"disappointed\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude1e <\/span><span class=\"reaction-description font-medium\">Disappointed <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"inaccurate\"><span class=\"reaction-count\" data-reaction-count=\"inaccurate\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\u274c <\/span><span class=\"reaction-description font-medium\">Inaccurate <\/span><\/button><\/div><div id=\"reaction-message\" class=\"hidden mt-8 rounded-md border border-primary-100 bg-white\/50 backdrop-blur-sm px-4 py-2 text-sm text-center shadow-sm\"><\/div><\/section><div class=\"calcdash-ad not-prose calcdash-ad--calc_result calcdash-ad--in-article\"><ins class=\"adsbygoogle\" style=\"display:block\" data-ad-client=\"ca-pub-1721569815777345\" data-ad-slot=\"6317458308\" data-ad-layout=\"in-article\" data-ad-format=\"fluid\"><\/ins><script>(adsbygoogle = window.adsbygoogle || []).push({});<\/script><\/div>\n<style id=\"wpcoderpro-inline-1\">.formula-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section class=\"formula-box\" aria-labelledby=\"formula-title\">\r\n\t<div id=\"formula-title\" class=\"not-prose font-display mb-4\"><span class=\"icon icon-formula\" aria-hidden=\"true\"><\/span><h2>FENa Formula<\/h2><\/div>\r\n\t<figure class=\"not-prose formula\">\r\n\t\t<figcaption class=\"formula-title\">Formula<\/figcaption>\r\n\t\t<div class=\"text-base text-gray-800\" id=\"formula\">\n\\[ \\text{FENa (\\%)} = \\frac{U_{\\text{Na}} \\times S_{\\text{Cr}}}{S_{\\text{Na}} \\times U_{\\text{Cr}}} \\times 100 \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(U_{\\text{Na}}\\) \u2014 urine sodium concentration (mmol\/L, same numeric value in mEq\/L)<\/li>\n<li>\\(S_{\\text{Na}}\\) \u2014 serum sodium concentration (mmol\/L)<\/li>\n<li>\\(U_{\\text{Cr}}\\) \u2014 urine creatinine concentration (mg\/dL or \u00b5mol\/L)<\/li>\n<li>\\(S_{\\text{Cr}}\\) \u2014 serum creatinine concentration (mg\/dL or \u00b5mol\/L)<\/li>\n<\/ul>\n<p>The formula compares the amount of sodium the kidneys clear relative to the amount of creatinine they clear, expressed as a percentage of the filtered sodium load. Because the urine and serum terms of each solute appear as ratios, any consistent concentration units cancel out.<\/p>\n<p>\r\n<\/section>\n<style id=\"wpcoderpro-inline-459\">.calculation-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section id=\"calculation\" class=\"calculation-box\" aria-labelledby=\"calculation-title\">\r\n\t<div id=\"calculation-title\" class=\"not-prose font-display\"><span class=\"icon icon-unordered-list\" aria-hidden=\"true\"><\/span><h2>FENa \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> a patient with acute kidney injury has a urine sodium of 40 mmol\/L, a serum sodium of 138 mmol\/L, a urine creatinine of 60 mg\/dL and a serum creatinine of 2 mg\/dL.<\/p>\n<p>\\[ \\text{FENa} = \\frac{40 \\times 2}{138 \\times 60} \\times 100 = \\frac{80}{8280} \\times 100 \\approx 0.97\\% \\]<\/p>\n<p><strong>Result:<\/strong> FENa \u2248 0.97%, which is below 1% and suggests a prerenal (hemodynamic) cause of acute kidney injury, such as volume depletion or heart failure.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the FENa Calculator<\/h2>\n<ol>\n<li>Enter the <strong>Urine sodium<\/strong> value from a spot (random) urine sample in mmol\/L (the number is identical in mEq\/L).<\/li>\n<li>Enter the <strong>Serum sodium<\/strong> value from the blood drawn at roughly the same time, in mmol\/L.<\/li>\n<li>Enter the <strong>Urine creatinine<\/strong> value and select its unit \u2014 mg\/dL or \u00b5mol\/L \u2014 using the Unit dropdown next to the field.<\/li>\n<li>Enter the <strong>Serum creatinine<\/strong> value from the same time point and select its unit the same way.<\/li>\n<li>Read your <strong>Fractional excretion of sodium<\/strong> in percent, together with an interpretation (prerenal, indeterminate, or intrinsic renal injury) and a short explanation of what the result means.<\/li>\n<\/ol>\n<p>If you enter creatinine in \u00b5mol\/L, the calculator automatically converts it to mg\/dL (1 mg\/dL = 88.4 \u00b5mol\/L), so both units give the same FENa.<\/p>\n<h2>What Is the Fractional Excretion of Sodium?<\/h2>\n<p>FENa is the percentage of the sodium filtered by the kidneys that ends up being excreted in the urine. In a healthy person on a normal diet, the kidneys reabsorb more than 99% of filtered sodium, so FENa is typically well below 1%. The classic clinical use of FENa is to help distinguish between the two major categories of acute kidney injury (AKI): prerenal and intrinsic renal.<\/p>\n<p>In a <strong>prerenal<\/strong> state \u2014 volume depletion, hemorrhage, heart failure, cirrhosis \u2014 the kidney itself is intact. It senses reduced perfusion and mounts a maximal sodium-conserving response, driving FENa below 1%. In <strong>intrinsic<\/strong> injury, most commonly acute tubular necrosis (ATN) from ischemia, sepsis or nephrotoxins, the tubular cells are damaged and can no longer reabsorb sodium effectively, so FENa rises above 2%. Values between 1% and 2% are considered indeterminate and must be interpreted with the wider clinical picture.<\/p>\n<p>A low FENa suggests that renal function may improve rapidly once the underlying hemodynamic problem is corrected, for example with intravenous fluids. A high FENa points toward intrinsic damage, where management is supportive \u2014 removing the offending drug, treating sepsis, and maintaining electrolyte and fluid balance \u2014 rather than aggressive volume expansion. FENa is a bedside clue, not a diagnosis; it should always be weighed together with the history, urine output, urinalysis, and the response to treatment.<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<ul>\n<li><strong>Diuretics invalidate FENa.<\/strong> Loop and thiazide diuretics force sodium into the urine and can push FENa above 2% even when the true state is prerenal. If the patient has received diuretics recently, prefer the fractional excretion of urea (FEUrea), where a value below 35% suggests a prerenal cause.<\/li>\n<li><strong>Chronic kidney disease distorts the result.<\/strong> In CKD the baseline FENa is already elevated, so a \"high\" value does not necessarily indicate acute tubular necrosis. Check the result against the patient's baseline kidney function \u2014 our <a href=\"https:\/\/wpcalc.com\/en\/medical\/egfr-calculator-ckd-epi-2021\/\">eGFR calculator (CKD-EPI 2021)<\/a> can help estimate it.<\/li>\n<li><strong>Some specific conditions give misleadingly low values.<\/strong> Contrast nephropathy, rhabdomyolysis and glomerulonephritis can produce a low FENa despite intrinsic injury.<\/li>\n<li><strong>Timing and stability matter.<\/strong> FENa is most reliable in oliguric AKI with stable (not rapidly changing) kidney function, using a urine and blood sample collected at the same time point.<\/li>\n<li><strong>Hyponatremia or hypernatremia?<\/strong> Interpret the serum sodium itself alongside FENa; a corrected value may be needed in hyperglycemia \u2014 see the <a href=\"https:\/\/wpcalc.com\/en\/medical\/hyperglycemia-sodium-correction\/\">corrected sodium calculator<\/a>. For a broader metabolic picture, the <a href=\"https:\/\/wpcalc.com\/en\/medical\/serum-osmolality-calculator\/\">serum osmolality calculator<\/a> is a useful companion, and in liver disease the <a href=\"https:\/\/wpcalc.com\/en\/medical\/meld-na-score-calculator\/\">MELD-Na score<\/a> incorporates sodium into prognosis.<\/li>\n<\/ul>\n<p>The thresholds of less than 1% and greater than 2% come from the classic studies by Espinel (JAMA, 1976) and Miller et al. (Annals of Internal Medicine, 1978). This calculator is provided for information and education only and is not a substitute for clinical judgment or a medical diagnosis.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Estimate the fractional excretion of sodium (FENa) from urine and serum sodium and creatinine values, and see whether your result points to a prerenal or an intrinsic cause of acute kidney injury.<\/p>\n","protected":false},"author":3168,"featured_media":0,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[8],"tags":[50],"class_list":["post-66655","post","type-post","status-publish","format-standard","hentry","category-medical","tag-diagnostics"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66655","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/users\/3168"}],"replies":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/comments?post=66655"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66655\/revisions"}],"predecessor-version":[{"id":66656,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66655\/revisions\/66656"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66655"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66655"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66655"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}