diff --git a/src/main/scala/bad/robot/temperature/ErrorOnTemperatureSpike.scala b/src/main/scala/bad/robot/temperature/ErrorOnTemperatureSpike.scala
index 91ce684..ba80669 100644
--- a/src/main/scala/bad/robot/temperature/ErrorOnTemperatureSpike.scala
+++ b/src/main/scala/bad/robot/temperature/ErrorOnTemperatureSpike.scala
@@ -9,8 +9,7 @@ object ErrorOnTemperatureSpike {
 
   private val spikePercentage = 30
 
-  // negative numbers would be a decrease, which we'll ignore (use Math.abs if we change our mind later)
-  def percentageIncrease(oldValue: Double, newValue: Double): Double = (newValue - oldValue) / oldValue * 100
+  def percentageIncrease(oldValue: Double, newValue: Double): Double = Math.abs((newValue - oldValue) / oldValue * 100)
 
   /**
     * @param delegate delegate writer
diff --git a/src/test/scala/bad/robot/temperature/ErrorOnTemperatureSpikeTest.scala b/src/test/scala/bad/robot/temperature/ErrorOnTemperatureSpikeTest.scala
index 3bbe9fb..d14ffa6 100644
--- a/src/test/scala/bad/robot/temperature/ErrorOnTemperatureSpikeTest.scala
+++ b/src/test/scala/bad/robot/temperature/ErrorOnTemperatureSpikeTest.scala
@@ -74,18 +74,17 @@ class ErrorOnTemperatureSpikeTest extends Specification {
     )
   }
 
-  "Doesn't error on negative spiked values (single sensor)" >> {
+  "Negative spikes values (single sensor)" >> {
     val delegate = new StubWriter
     val writer = new ErrorOnTemperatureSpike(delegate)
     writer.write(Measurement(Host("example"), Seconds(1), List(SensorReading("A", Temperature(21.1)))))
     writer.write(Measurement(Host("example"), Seconds(2), List(SensorReading("A", Temperature(21.4)))))
     writer.write(Measurement(Host("example"), Seconds(3), List(SensorReading("A", Temperature(21.6)))))
-    writer.write(Measurement(Host("example"), Seconds(4), List(SensorReading("A", Temperature(1.1))))) must be_\/-
+    writer.write(Measurement(Host("example"), Seconds(4), List(SensorReading("A", Temperature(1.1))))) must be_-\/.like(SensorError)
     delegate.temperatures must_== List(
       Measurement(Host("example"), Seconds(1), List(SensorReading("A", Temperature(21.1)))),
       Measurement(Host("example"), Seconds(2), List(SensorReading("A", Temperature(21.4)))),
       Measurement(Host("example"), Seconds(3), List(SensorReading("A", Temperature(21.6)))),
-      Measurement(Host("example"), Seconds(4), List(SensorReading("A", Temperature(1.1))))
     )
   }