Index: firmware/App/Modes/BloodPrime.c
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diff -u -rac6532c81f2a6d4ad1c67420c22d59f6aeeaae13 -rdb22e9d530c5c9d62b02e6dcc72ed96fb67c165a
--- firmware/App/Modes/BloodPrime.c	(.../BloodPrime.c)	(revision ac6532c81f2a6d4ad1c67420c22d59f6aeeaae13)
+++ firmware/App/Modes/BloodPrime.c	(.../BloodPrime.c)	(revision db22e9d530c5c9d62b02e6dcc72ed96fb67c165a)
@@ -67,7 +67,7 @@
 static U32  bloodPrimePublishTimerCtr;                                      ///< Timer counter for determining interval for blood prime status to be published.
 
 static F32  cumulativeBloodPrimeVolume_mL;                                  ///< Total cumulative blood prime volume (in mL).
-static U32  bloodPrimeMotorCount;                                           ///< The cumulative sum of BP motor encoder counts used for independent blood prime volume check.
+static S32  bloodPrimeMotorCount;                                           ///< The cumulative sum of BP motor encoder counts used for independent blood prime volume check.
 static U32  bloodPrimeLastMotorCount;                                       ///< The last BP motor encoder count read for independent blood prime volume check.
 static F32  bloodPrimeVolumeDelivered_Safety;                               ///< The cumulative independent blood prime volume (in mL) calculated so far.
 
@@ -209,7 +209,7 @@
     // update blood prime volume delivered so far
     cumulativeBloodPrimeVolume_mL += ( getMeasuredBloodFlowRate() * BLOOD_PRIME_FLOW_INTEGRATOR );
     // update independent calculated safety volume delivered so far
-    bloodPrimeMotorCount = u32DiffWithWrap( bloodPrimeLastMotorCount, getBloodPumpMotorCount() ) / BP_HALL_EDGE_COUNTS_PER_REV;
+    bloodPrimeMotorCount = u32BiDiffWithWrap( bloodPrimeLastMotorCount, getBloodPumpMotorCount() ) / BP_HALL_EDGE_COUNTS_PER_REV;
     bloodPrimeVolumeDelivered_Safety = ( (F32)bloodPrimeMotorCount * VOLUME_PER_BP_MOTOR_REV_ML );  // TODO - include upstream pressure compensation to this calc
 
     // Has blood prime completed?