diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 20b178b71..e5c4ab096 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -6152,7 +6152,7 @@ model LabResult {
   sourceReferenceText String?   @map("source_reference_text")
   /// @pending-consumer(#1095) v1.42 — the value as printed on the source
   /// report, kept when a write converted it into the biomarker's canonical
-  /// unit. NULL when no conversion happened. A later edit of `value` clears it.
+  /// unit. NULL when no conversion happened. An edit that changes `value` clears it.
   sourceValue         Float?    @map("source_value")
   /// @pending-consumer(#1095) v1.42 — the unit the source report printed,
   /// paired with `sourceValue`.
diff --git a/src/app/api/labs/[id]/route.ts b/src/app/api/labs/[id]/route.ts
index 6ef7f3d77..e8363b6e5 100644
--- a/src/app/api/labs/[id]/route.ts
+++ b/src/app/api/labs/[id]/route.ts
@@ -195,8 +195,14 @@ export const PUT = apiHandler(
     if (d.value !== undefined) {
       data.value = d.value;
       // A number the person typed is no longer the conversion of what the
-      // report printed, so the "converted from" record would now be wrong.
-      if (existing.sourceValue !== null || existing.sourceUnit !== null) {
+      // report printed, so the "converted from" record would now be wrong. Only
+      // a CHANGED value says so: a client that sends the whole form back with
+      // the value untouched (the web edit dialog does) must not erase the
+      // record. Both sides are plain numbers (`value` is a Float column).
+      if (
+        d.value !== existing.value &&
+        (existing.sourceValue !== null || existing.sourceUnit !== null)
+      ) {
         data.sourceValue = null;
         data.sourceUnit = null;
       }
diff --git a/src/components/labs/lab-history-list.tsx b/src/components/labs/lab-history-list.tsx
index 6e581f259..109d32de6 100644
--- a/src/components/labs/lab-history-list.tsx
+++ b/src/components/labs/lab-history-list.tsx
@@ -32,6 +32,7 @@ import { localizedApiError } from "@/lib/api/localized-error";
 import { formatLabReading } from "@/lib/labs/format-value";
 import { LabConvertedNote } from "./lab-converted-note";
 import { resolveNoteForUpdate } from "@/lib/labs/note-update";
+import { resolveValueForUpdate, valueEditSeed } from "@/lib/labs/value-update";
 import { useTranslations } from "@/lib/i18n/context";
 import { queryKeys } from "@/lib/query-keys";
 
@@ -177,7 +178,8 @@ export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
   const updateMutation = useMutation({
     mutationFn: (body: {
       id: string;
-      // Exactly one of value / valueText is set, matching the row's type.
+      // At most one of value / valueText is set, matching the row's type;
+      // null leaves the field out of the PUT (an untouched value).
       value: number | null;
       valueText: string | null;
       takenAt: string;
@@ -219,7 +221,7 @@ export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
 
   async function openEdit(reading: LabResultDto) {
     const seed = {
-      value: reading.value !== null ? String(reading.value) : "",
+      value: reading.value !== null ? valueEditSeed(reading.value) : "",
       valueText: reading.valueText ?? "",
       takenAt: toDateTimeLocal(reading.takenAt),
       note: "",
@@ -275,11 +277,20 @@ export function LabHistoryList({ readings }: { readings: LabResultDto[] }) {
       }
       qualitativeValue = trimmed;
     } else {
-      numericValue = parseDecimal(editValue);
-      if (numericValue === null) {
+      const parsed = parseDecimal(editValue);
+      if (parsed === null) {
         setEditError(t("labs.form.requiredError"));
         return;
       }
+      // An untouched field is left out of the PUT (null here), so the stored
+      // full-precision value and a conversion record survive a date or note
+      // edit.
+      numericValue =
+        resolveValueForUpdate({
+          seed: editSeed.value,
+          editorValue: editValue,
+          parsed,
+        }) ?? null;
     }
     const takenDate = new Date(editTakenAt);
     if (Number.isNaN(takenDate.getTime())) {
diff --git a/src/lib/labs/__tests__/value-update.test.ts b/src/lib/labs/__tests__/value-update.test.ts
new file mode 100644
index 000000000..dcc98d4c8
--- /dev/null
+++ b/src/lib/labs/__tests__/value-update.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from "vitest";
+
+import { resolveValueForUpdate, valueEditSeed } from "../value-update";
+
+describe("valueEditSeed", () => {
+  it("opens a converted reading at display precision, not its stored tail", () => {
+    // 5.6 mmol/L of glucose, stored in mg/dL.
+    expect(valueEditSeed(100.90191999999999)).toBe("100.9");
+  });
+
+  it("keeps whole numbers bare and at most two decimals", () => {
+    expect(valueEditSeed(110)).toBe("110");
+    expect(valueEditSeed(4.567)).toBe("4.57");
+  });
+});
+
+describe("resolveValueForUpdate", () => {
+  it("leaves an untouched value out of the update", () => {
+    expect(
+      resolveValueForUpdate({
+        seed: "100.9",
+        editorValue: "100.9",
+        parsed: 100.9,
+      }),
+    ).toBeUndefined();
+  });
+
+  it("sends a changed value", () => {
+    expect(
+      resolveValueForUpdate({
+        seed: "100.9",
+        editorValue: "101,5",
+        parsed: 101.5,
+      }),
+    ).toBe(101.5);
+  });
+
+  it("sends a value typed back to the same number in another spelling", () => {
+    // "100,9" is a deliberate re-entry; the stored tail goes, as for any edit.
+    expect(
+      resolveValueForUpdate({
+        seed: "100.9",
+        editorValue: "100,9",
+        parsed: 100.9,
+      }),
+    ).toBe(100.9);
+  });
+});
diff --git a/src/lib/labs/value-update.ts b/src/lib/labs/value-update.ts
new file mode 100644
index 000000000..5cbbad305
--- /dev/null
+++ b/src/lib/labs/value-update.ts
@@ -0,0 +1,29 @@
+import { formatLabValue } from "./format-value";
+
+/**
+ * The numeric value of a lab reading in the edit sheet.
+ *
+ * A converted reading is stored at full precision (5.6 mmol/L of glucose is
+ * 100.90191999999999 mg/dL), and the sheet must not open with that tail. The
+ * field starts from the value as every lab surface shows it: whole numbers
+ * bare, at most two decimals. Written with a dot, which the sheet's parser
+ * reads, rather than in the reader's locale, which an input cannot round-trip.
+ */
+export function valueEditSeed(value: number): string {
+  return formatLabValue(value, String);
+}
+
+/**
+ * What the edit PUT sends for the value. A field the person did not touch is
+ * omitted (`undefined`), so the stored full-precision number stays as it is and
+ * a converted reading keeps its "converted from" record: sending the rounded
+ * start value back would both lose precision and read to the server as a new
+ * number. A changed field sends the parsed number.
+ */
+export function resolveValueForUpdate(args: {
+  seed: string;
+  editorValue: string;
+  parsed: number;
+}): number | undefined {
+  return args.editorValue.trim() === args.seed.trim() ? undefined : args.parsed;
+}
diff --git a/tests/integration/labs-unit-conversion.test.ts b/tests/integration/labs-unit-conversion.test.ts
index 9634476ac..89af81ce8 100644
--- a/tests/integration/labs-unit-conversion.test.ts
+++ b/tests/integration/labs-unit-conversion.test.ts
@@ -14,7 +14,8 @@
  *     the printed unit is in range after conversion;
  *   - the OCR commit converts, and the same scan committed again is a duplicate
  *     (the stored number is converted, so the printed one is what matches);
- *   - editing the value of a converted reading clears the conversion record;
+ *   - changing the value of a converted reading clears the conversion record,
+ *     and an edit that sends the value back unchanged keeps it;
  *   - the list returns the fields.
  */
 import { NextRequest } from "next/server";
@@ -338,10 +339,24 @@ describe("editing and reading back", () => {
       takenAt: "2026-07-02T08:00:00.000Z",
     });
     const id = created.json.data!.id;
-    const edited = await call<Dto>("one", "PUT", { note: "fasting" }, id);
+    const storedValue = created.json.data!.value;
+    // The web edit dialog sends the form back whole, the unchanged value
+    // included; only a changed value clears the record.
+    const edited = await call<Dto>(
+      "one",
+      "PUT",
+      {
+        value: storedValue,
+        takenAt: "2026-07-02T09:00:00.000Z",
+        note: "fasting",
+      },
+      id,
+    );
     expect(edited.status).toBe(200);
+    expect(edited.json.data!.value).toBe(storedValue);
     expect(edited.json.data!.converted).toBe(true);
     expect(edited.json.data!.sourceValue).toBe(6.1);
+    expect(edited.json.data!.sourceUnit).toBe("mmol/L");
   });
 
   it("returns the fields on the list and on the single reading", async () => {